type CreateUnion<
  N extends number,
  T extends any[] = [any],
  S = `${T['length']}` extends `${number}${number}` ? `${T['length']}` : `0${T['length']}`
> = N extends T['length'] ? S : S | CreateUnion<N, [...T, any]>

type ValidDate<T extends string> = T extends `${infer A}${infer B}${infer C}${infer D}`
  ? `${A}${B}` extends CreateUnion<12>
    ? `${A}${B}` extends '02' ? `${C}${D}` extends CreateUnion<28> ? true : false
      : `${A}${B}` extends '01' | '03' | '05' | '07' | '08' | '10' | '12' ? `${C}${D}` extends CreateUnion<31> ? true : false
      : `${C}${D}` extends CreateUnion<30> ? true : false
    : false
  : false

Solution by 2083335157 #35084

type AddZeroPadding<S extends string> = S extends `${1 | 2 | 3}${number}` ? S : `0${S}`;

type Range<
  End extends number,
  Result extends string = never,
  Counter extends unknown[] = [unknown]
> = Counter["length"] extends End
  ? Result | AddZeroPadding<`${Counter["length"]}`>
  : Range<End, Result | AddZeroPadding<`${Counter["length"]}`>, [...Counter, unknown]>;

type ValidDate<T extends string> = T extends
  | `${"01" | "03" | "05" | "07" | "08" | "11" | "12"}${Range<31>}`
  | `${"04" | "06" | "09" | "11"}${Range<30>}`
  | `02${Range<28>}`
  ? true
  : false;

Solution by yukicountry #34454

type Ends = ['29' | '30', '29' | '30' | '31', never];
type D<E extends number> = `${0 | 1 | 2}${1 | 2 | 3 | 4 | 5 | 6 | 7 | 8}` | '09' | '19' | Ends[E];

type All = `01${D<1>}` | `02${D<2>}` | `03${D<1>}` | `04${D<0>}` | `05${D<1>}` | `06${D<0>}` |
           `07${D<1>}` | `08${D<1>}` | `09${D<0>}` | `10${D<1>}` | `11${D<0>}` | `12${D<1>}`;

type ValidDate<T extends string> = T extends All ? true : false;

Solution by alexandroppolus #33508

type Days31 =
  | "01"
  | "02"
  | "03"
  | "04"
  | "05"
  | "06"
  | "07"
  | "08"
  | "09"
  | "10"
  | "11"
  | "12"
  | "13"
  | "14"
  | "15"
  | "16"
  | "17"
  | "18"
  | "19"
  | "20"
  | "21"
  | "22"
  | "23"
  | "24"
  | "25"
  | "26"
  | "27"
  | "28"
  | "29"
  | "30"
  | "31";

type MonthDayMap = {
  "01": Days31;
  "02": Exclude<Days31, "29" | "30" | "31">;
  "03": Days31;
  "04": Exclude<Days31, "31">;
  "05": Days31;
  "06": Exclude<Days31, "31">;
  "07": Days31;
  "08": Days31;
  "09": Exclude<Days31, "31">;
  "10": Days31;
  "11": Exclude<Days31, "31">;
  "12": Days31;
};
type ValidDate<T extends string> = T extends `${keyof MonthDayMap}${infer Day}`
  ? T extends `${infer Month}${Day}`
    ? Day extends MonthDayMap[Month & keyof MonthDayMap]
      ? true
      : false
    : false
  : false;

Solution by Vampirelee #32662

EZ.

// your answers
type Int = '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' | '0';
type February = '02'
type MonthsHasDays30 = '04' | '06' | '09' | '11';
type MonthsHasDays31 = '01' | '03' | '05' | '07' | '08' | '10' | '12';
type Months = February | MonthsHasDays30 | MonthsHasDays31;
type Days = Exclude<`${0 | 1 | 2}${Int}` | '30' | '31', '00'>

type Decimal = `${Int}${Int}`;

type DaysByMonth<T extends February | MonthsHasDays30 | MonthsHasDays31> = 
  T extends February 
    ? Exclude<Days, '29' | '30' | '31'> 
    : T extends MonthsHasDays30 
      ? Exclude<Days, '30'>
      : Days

type AllDays = {
  [M in Months]: `${M}${DaysByMonth<M>}`
}[Months]

type ValidDate<T extends string> = T extends AllDays ? true : false;

Solution by kakasoo #32605

type GreaterThan<
  T extends number,
  U extends number,
  R extends any[] = []
> = T extends R["length"]
  ? false
  : U extends R["length"]
  ? true
  : GreaterThan<T, U, [...R, 1]>;

type toNumber<T extends string> = T extends `0${infer R}`
  ? toNumber<R>
  : T extends `${infer F extends number}`
  ? F
  : 0;

type DaysLimit<T extends number> = [never,32,29,32,31,32,31,32,32,31,32,31,32][T];

type ValidDate<T extends string> = T extends `${infer M1}${infer M2}${infer D}`
  ? [
      GreaterThan<13, toNumber<`${M1}${M2}`>>,
      GreaterThan<toNumber<D>, 0>,
      GreaterThan<DaysLimit<toNumber<`${M1}${M2}`>>, toNumber<D>>
    ] extends [true, true, true]
    ? true
    : false
  : false;

Solution by vangie #32339

type MonthValue = 0 | 1
type DecimalValue = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
type DayValue = 0 | 1 | 2 | 3

type ValidDate<
  T extends string
> = `${T}` extends `${MonthValue}${DecimalValue}${DayValue}${DecimalValue}` 
  ? `${T}` extends `00${string}` 
    | `${string}00` 
    | `1${Exclude<DecimalValue, 0 | 1 | 2>}${string}` 
    | `${string}3${Exclude<DecimalValue, 1>}` 
    | `0229` 
      ? false 
      : true 
  : false

Solution by simone-paglino #29652

// 数字的范围
type Num = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

type D = 0 | 1 | 2
// 月小
type MinMM = `0${4 | 6 | 7}` | `11`
// 月大
type MaxMM = Exclude<`0${Num}` | `1${D}`, MinMM | `02`>

type Day = `${D}${Num}` | `3${Exclude<D, 2>}`

type ValidDate<T extends string> = T extends `${MaxMM}${Day}` | `${MinMM}${Exclude<Day, `31`>}` | `02${Exclude<Day, `29` | `30` | `31`>}` ? true : false

Solution by wuxin0011 #27378

9155 - ValidDate

I found this challenge to be quite fun. It's a problem that has a few vastly different ways to approach it, which ends up being a good lesson on how to balance keeping things terse and making it easy to read. It's fun to see what TypeScript can do! Even parsing dates.. hah.

🎥 Video Explanation

🔢 Code

// ============= Test Cases =============
import type { Equal, Expect } from './test-utils'

type A1 = ValidDate<'0102'>;
type B1 = true;
type C1 = Expect<Equal<A1, B1>>;

type A2 = ValidDate<'0131'>;
type B2 = true;
type C2 = Expect<Equal<A2, B2>>;

type A3 = ValidDate<'1231'>;
type B3 = true;
type C3 = Expect<Equal<A3, B3>>;

type A4 = ValidDate<'0229'>;
type B4 = false;
type C4 = Expect<Equal<A4, B4>>;

type A5 = ValidDate<'0100'>;
type B5 = false;
type C5 = Expect<Equal<A5, B5>>;

type A6 = ValidDate<'0132'>;
type B6 = false;
type C6 = Expect<Equal<A6, B6>>;

type A7 = ValidDate<'1301'>;
type B7 = false;
type C7 = Expect<Equal<A7, B7>>;

type A8 = ValidDate<'0123'>;
type B8 = true;
type C8 = Expect<Equal<A8, B8>>;

type A9 = ValidDate<'01234'>;
type B9 = false;
type C9 = Expect<Equal<A9, B9>>;

type A10 = ValidDate<''>;
type B10 = false;
type C10 = Expect<Equal<A10, B10>>;

// ============= Your Code Here =============
type Day = {
  '01': '31';
  '02': '28';
  '03': '31';
  '04': '30';
  '05': '31';
  '06': '30';
  '07': '31';
  '08': '31';
  '09': '30';
  '10': '31';
  '11': '30';
  '12': '31';
};
type Month = keyof Day;

type RemoveZero<T> =
  T extends `0${infer R}`
  ? RemoveZero<R>
  : T;

type CheckDay<
  Day extends string,
  DaysThisMonth extends string,
  DayWithoutZero extends string = RemoveZero<Day>,
  Count extends 1[] = []
> =
  DayWithoutZero extends ''
  ? false
  : `${Count['length']}` extends DayWithoutZero
    ? true
    : `${Count['length']}` extends DaysThisMonth
      ? false
      : CheckDay<
          Day,
          DaysThisMonth,
          DayWithoutZero,
          [...Count, 1]
        >;

type ValidDate<T extends string> =
  T extends `${infer M1}${infer M2}${infer Tail}`
  ? `${M1}${M2}` extends Month
    ? CheckDay<Tail, Day[`${M1}${M2}`]>
    : false
  : false;

// ============== Alternatives ==============
// @teamchong
type _1_9 = 1|2|3|4|5|6|7|8|9;
type _0_9 = 0|1|2|3|4|5|6|7|8|9;
type _0_8 = 0|1|2|3|4|5|6|7|8;
type D30 = `0${_1_9}`|`1${_0_9}`|`2${_0_9}`|`30`;
type D31 = `0${_1_9}`|`1${_0_9}`|`2${_0_9}`|`30`|`31`;
type D28 = `0${_1_9}`|`1${_0_9}`|`2${_0_8}`;
type M31 = `${`01`|`03`|`05`|`07`|`08`|`10`|`12`}${D31}`;
type M30 = `${`04`|`06`|`09`|`11`}${D30}`;
type M28 = `02${D28}`;
type ValidDate<T> =
  T extends M28 | M30 | M31
  ? true
  : false;

// @LoTwT
type Num = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;
type MM = `0${Num}` | `1${0 | 1 | 2}`;
type AllDate =
    // All months have 0-9 days
  | `${MM}${`${0}${Num}`

    // All months have 10-19 days
  | `${1}${0 | Num}`

    // February
  | `2${0 | Exclude<Num, 9>}`}`
  
    // Non-February months ending with 30 days
    | `${Exclude<MM, '02'>}${29 | 30}`
    
    // Add the 31th days for those months that have it
  | `${Exclude<MM, '02' | '04' | '06' | '09' | '11'>}${31}`;

type ValidDate<T> =
  T extends AllDate
  ? true
  : false;

// @jiangshanmeta
type _1_9 = '1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9';
type _0_9 = '0'|_1_9;
type Thirty =
  | `0${_1_9}`
  | `1${_0_9}`
  | `2${_0_9}`
  | '30';
type ThirtyOne = Thirty | '31';
type TwentyEight = Exclude<Thirty,'30' | '29'>;
type DateMap = {
  '01':ThirtyOne;
  '02':TwentyEight;
  '03':ThirtyOne;
  '04':Thirty;
  '05':ThirtyOne;
  '06':Thirty;
  '07':ThirtyOne;
  '08':ThirtyOne;
  '09':Thirty;
  '10':ThirtyOne;
  '11':Thirty;
  '12':ThirtyOne;
};
type ValidDate<T extends string> =
  T extends `${infer F}${infer S}${infer Tail}`
  ? `${F}${S}` extends keyof DateMap
    ? Tail extends DateMap[`${F}${S}`]
      ? true
      : false
    : false
  : false;

➕ More Solutions

For more video solutions to other challenges: see the umbrella list! https://github.com/type-challenges/type-challenges/issues/21338

Solution by dimitropoulos #25355

type Digit = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;
type Month = Exclude<Digit, 0> | 10 | 11 | 12;
type LargeMonth = Exclude<Month, 2 | 4 | 6 | 9 | 11>;
type ValidDay<Month extends number> =
  `0${Exclude<Digit, 0>}` |     //01~09
  `${1 | 2}${Month extends 2 ? Exclude<Digit, 9> : Digit}` |    //10~28 | 10~29
  `${Month extends 2 ? never : Month extends LargeMonth ? 31 : 30}`;    //never | 30 | 31
type Number<T extends string | number> = T extends `${infer F}${infer R}` ? (F extends `0` ? Number<R> : (T extends `${infer N extends number}` ? N : T)) : T;

type ValidDate<T extends string> = T extends `${infer M1 extends number}${infer M2 extends number}${infer D1 extends number}${infer D2 extends Digit}` ?
  (Number<`${M1}${M2}`> extends Month ?
    (`${D1}${D2}` extends ValidDay<Number<`${M1}${M2}`>> ? true : false)
    : false)
  : false;

Solution by E-uler #25109

type Months = '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12'
type DaysFebruary = Months | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28'
type DaysFullMonth = DaysFebruary | '29' | '30' | '31'


type ValidDate<T extends string> = T extends `${infer Month extends Months}${DaysFullMonth}`
? Month extends '02' 
  ? T extends `${Months}${DaysFebruary}`
    ? true
    : false
  : true
: false

Solution by NeylonR #24536

// your answers

type Base = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

type FillZero<T extends string | number> = `${T}` extends `${Base}` ? `0${T}`: T

type toTuple<T extends number, R extends any[] = []> = R['length'] extends T ? R : toTuple<T, [...R, 1]>

type NumberRange<
  L extends number,
  H extends number,
  R extends any[] = toTuple<L>,
  Set extends any = never
> = R['length'] extends H
  ? Set | FillZero<H>
  : NumberRange<L, H, [...R, 1], Set | FillZero<R['length']>>

type Dates<> = {
  [K in NumberRange<1, 12>] : `${K}${NumberRange<1, 31>}`
}

type Special = "0229" | "0230" | `${"02" | "04" | "06" | "09" | "11"}${31}`

type ValidDate<T extends string> = T extends Exclude<Dates[keyof Dates], Special>
  ? true
  : false

Solution by jxhhdx #24513

// your answers
type DaysOfFeb = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28
type DaysOfSM = DaysOfFeb | 29 | 30
type DaysOfBM = DaysOfSM | 31

type MonthDays = {
  1: DaysOfBM,
  2: DaysOfFeb,
  3: DaysOfBM,
  4: DaysOfSM,
  5: DaysOfBM,
  6: DaysOfSM,
  7: DaysOfBM,
  8: DaysOfBM,
  9: DaysOfSM,
  10: DaysOfBM,
  11: DaysOfSM,
  12: DaysOfBM,
}

type Months = keyof MonthDays

type Format<T extends number | string> = 
  T extends unknown
  ? `${T}` extends `${1|2|3|4|5|6|7|8|9}`
    ? `0${T}`
    : `${T}`
  : never

// enum all
type ValidDates<M extends Months = Months> = 
  M extends unknown 
  ? `${Format<M>}${Format<MonthDays[M]>}`
  : never

type ValidDate<T extends string> = T extends ValidDates ? true : false

Solution by snakeUni #24067

type StringToNumber<T extends string | number> =
  `${T}` extends `0${infer R extends number}`
    ? StringToNumber<R>
    : `${T}` extends `${infer R extends number}`
      ? R
      : never

type GreaterThanOrEqual<
  T extends number,
  U extends number,
  R extends unknown[] = [],
> = T extends U
  ? true
  : R['length'] extends T
    ? false
    : R['length'] extends U
      ? true
      : GreaterThanOrEqual<T, U, [...R, 0]>

interface Days {
  '01': 31
  '02': 28
  '03': 31
  '04': 30
  '05': 31
  '06': 30
  '07': 31
  '08': 31
  '09': 30
  '10': 31
  '11': 30
  '12': 31
}

type ValidDate<T extends string> =
  T extends `${infer M1}${infer M2}${infer D}`
    ? `${M1}${M2}` extends keyof Days
      ? GreaterThanOrEqual<StringToNumber<D>, 1>
      | GreaterThanOrEqual<Days[`${M1}${M2}`], StringToNumber<D>> extends true
        ? true
        : false
      : false
    : false

Solution by drylint #22975

type OneThroughNine = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

type GenerateLimit<Limit extends any[], Result extends string[]= [never]> = Result['length'] extends Limit['length'] ? Result[number] : Result['length'] extends OneThroughNine ? GenerateLimit<Limit, [...Result, 0${Result['length']}]> : GenerateLimit<Limit, [...Result, ${Result['length']}]>

type Days = GenerateLimit<[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]> type Months = GenerateLimit<[0,1,2,3,4,5,6,7,8,9,10,11,12]>

type ValidDate = T extends ${Months}${Days} ? T extends 0229 ? false :true :false

Solution by amcguire704 #22490

// toolkit to build number ranges as strings
type NTuple<N extends number, C extends unknown[]=[]> = C['length'] extends N ? C : NTuple<N, [...C, 0]>;
type Inc<N extends number> = [...NTuple<N>, 0]['length'];
type Subtract<T extends number, U extends number> = [...NTuple<T>] extends [...NTuple<U>, ...infer Rest] ? Rest['length'] : never;
type PaddedNumString<N extends number> = `${N}` extends `${infer F}${infer L}` ? L extends '' ? `0${F}` : `${N}` : never; 
type NumberRangeArray<Start extends number, End extends number> = Subtract<End, Start> extends 0 ? [PaddedNumString<End>] :
[PaddedNumString<Start>, ...NumberRangeArray<Inc<Start> extends number ? Inc<Start> : never, End>];

// the possible day values
type February = NumberRangeArray<1,28>[number];
type MinDayRange = February | NumberRangeArray<29,30>[number];
type MaxDayRange = MinDayRange | '31';

// month days are weird....
type YearMap = {
  '01': MaxDayRange,
  '02': February,
  '03': MaxDayRange,
  '04': MinDayRange,
  '05': MaxDayRange,
  '06': MinDayRange,
  '07': MaxDayRange,
  '08': MaxDayRange,
  '09': MinDayRange,
  '10': MaxDayRange,
  '11': MinDayRange,
  '12': MaxDayRange
}
type GetFromMap<T> = T extends keyof YearMap ? T : never;
type ValidDate<T extends string> = T extends `${infer M1}${infer M2}${infer D}` ? 
D extends YearMap[GetFromMap<`${M1}${M2}`>] ? true : false : false;

Solution by Karamuto #22227

type DaysOfFeb = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28
type DaysOfSM = DaysOfFeb | 29 | 30
type DaysOfBM = DaysOfSM | 31

type MonthDays = {
  1: DaysOfBM,
  2: DaysOfFeb,
  3: DaysOfBM,
  4: DaysOfSM,
  5: DaysOfBM,
  6: DaysOfSM,
  7: DaysOfBM,
  8: DaysOfBM,
  9: DaysOfSM,
  10: DaysOfBM,
  11: DaysOfSM,
  12: DaysOfBM,
}

type Months = keyof MonthDays

type Format<T extends number | string> = 
  T extends unknown
  ? `${T}` extends `${1|2|3|4|5|6|7|8|9}`
    ? `0${T}`
    : `${T}`
  : never

type ValidDates<M extends Months = Months> = 
  M extends unknown 
  ? `${Format<M>}${Format<MonthDays[M]>}`
  : never

type ValidDate<T extends string> = T extends ValidDates ? true : false

playground

中文笔记

Solution by zhaoyao91 #22010

// your answers
type Range = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
type WithOutZeroRange = '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
type FebDays = '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' 

type Getlen<T extends string,S extends any[] = []> = T extends `${infer F}${infer Rest}` ? 
Getlen<Rest,S extends [...infer R] ? [...R,1] : []> : S['length']

type MonthRange<T> = T extends `${infer F}${infer S}${infer Rest}` ? 
F extends '1' ?  S extends '0' | '1' | '2' ?  true : false : 
F extends '0' ? S extends Range ? true : false :true : false

type DayRange<T> = T extends `${infer F}${infer S}${infer Rest}` ?  
F extends '0' ? S extends WithOutZeroRange ?  true : false :
F extends  '1' ?  S extends Range ?  true : false :
F extends '2' ? S extends FebDays ? true : false:
F extends '3' ? S extends '0' | '1' ?  true : false : false : false

type DateRule<T extends string> = Getlen<T> extends 4 ?  
T extends `${infer F}${infer S}${infer Third}${infer Fourth}` ? 
Equal<MonthRange<`${F}${S}`>,DayRange<`${Third}${Fourth}`>> : false : false

type ValidDate<T extends string> = DateRule<T>

Solution by YqxLzx #21908

type Build<
  T extends number,
  Result extends Array<never> = []
> = Result["length"] extends T ? Result : Build<T, [...Result, never]>;
type EGT<
  T extends number,
  B extends number
> = Build<T> extends [...Build<B>, ...infer C]
    ? true
    : false

type RLT<
  T extends number,
  B extends number
> = Build<B> extends [...Build<T>, ...infer C]
    ? true
    : false

type ConvertNumber<T extends string> = T extends `0${infer C extends number}`
  ? C
  : T extends `${infer G extends number}`
  ? G
  : never;

type ValidDate<T extends string> =
T extends `0229` ? false :
T extends `${infer A}${infer B}${infer C}${infer D}` ?
  [
    EGT<ConvertNumber<`${A}${B}`>,1>,
    RLT<ConvertNumber<`${A}${B}`>,12>,
    EGT<ConvertNumber<`${C}${D}`>,1>,
    RLT<ConvertNumber<`${C}${D}`>,31>,
  ] extends [true,true,true,true] ? true : false: false

Solution by so11y #21327

type ThirtyDaysMonth = "04" | "06" | "09" | "11";
type ThirtyOneDaysMonth = "01" | "03" | "05" | "07" | "08" | "10" | "12";
type February = "02";

type GetDay<Arr extends any[], N extends number = Arr["length"]> = N extends
  | 1
  | 2
  | 3
  | 4
  | 5
  | 6
  | 7
  | 8
  | 9
  ? `0${N}`
  : `${N}`;

type Range<N, Arr extends string[] = ["01"]> = Arr["length"] extends N
  ? Arr[number]
  : Range<N, [...Arr, GetDay<[...Arr, any]>]>;

type ValidDays<Days, Max> = Days extends Range<Max> ? true : false;

type ValidDate<T extends string> = T extends `${February}${infer days}`
  ? ValidDays<days, 28>
  : T extends `${ThirtyDaysMonth}${infer days}`
  ? ValidDays<days, 30>
  : T extends `${ThirtyOneDaysMonth}${infer days}`
  ? ValidDays<days, 31>
  : false;

Solution by bedis-elacheche #21117

type Month =
  | '01'
  | '02'
  | '03'
  | '04'
  | '05'
  | '06'
  | '07'
  | '08'
  | '09'
  | '10'
  | '11'
  | '12';

type Day = {
  '01': '31';
  '02': '28';
  '03': '31';
  '04': '30';
  '05': '31';
  '06': '30';
  '07': '31';
  '08': '31';
  '09': '30';
  '10': '31';
  '11': '30';
  '12': '31';
};
type RemoveZero<T extends string> = T extends `0${infer R}` ? RemoveZero<R> : T;
type CheckDay<
  T extends string,
  M extends string,
  V extends string = RemoveZero<T>,
  R extends unknown[] = []
> = V extends ''
  ? false
  : `${R['length']}` extends V
  ? true
  : `${R['length']}` extends M
  ? false
  : CheckDay<T, M, V, [...R, 1]>;

type ValidDate<T extends string> = T extends `${infer M1}${infer M2}${infer R}`
  ? `${M1}${M2}` extends Month
    ? CheckDay<R, Day[`${M1}${M2}`]>
    : false
  : false;

Solution by bigcreate #20898

type Month31 = '01' | '03' | '05' | '07' | '08' | '10' | '12'
type Month30 = '04' | '06' | '09' | '11'
type Month28 = '02'

type _AllNumber = '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' | '0'

type ValidDate2<T extends string> =
  T extends `${ Month31 }${ infer Date1 }${ infer Date2 }`
    ? Date1 extends '0'
      ? Date2 extends '0' ? false : true // == __00
      : Date1 extends '1' | '2'
        ? Date2 extends _AllNumber ? true : false
        : Date1 extends '3'
          ? Date2 extends '0' | '1'
            ? true
            : false // > __32
          : false // > 4___
    : T extends `${ Month30 }${ infer Date1 }${ infer Date2 }`
      ? Date1 extends '0'
        ? Date2 extends '0' ? false : true // == __00
        : Date1 extends '1' | '2'
          ? Date2 extends _AllNumber ? true : false
          : Date1 extends '3'
            ? Date2 extends '0'
              ? true
              : false // > __31
            : false // 4___
      : T extends `${ Month28 }${ infer Date1 }${ infer Date2 }`
        ? Date1 extends '0'
          ? Date2 extends '0' ? false : true // == __00
          : Date1 extends '1'
            ? Date2 extends _AllNumber ? true : false
            : Date1 extends '2'
              ? Date2 extends '9'
                ? false // > 0229
                : true
              : false
        : false

#16935 is better.

Solution by lvjiaxuan #20651

type Day31 = '1' | '3' | '5' | '7' | '8' | '10' | '12';

type Day30 = '4' | '6' | '9' | '11';

type Day28 = '2';

type NumberToArray<N extends string, T extends any[] = []> = `${T['length']}` extends `${N}` ? T : NumberToArray<N, [...T, 1]>;

type GetNumber<S extends string> = S extends `${infer F}${infer R}` ? (F extends '0' ? R : `${F}${R}`) : `${S}`;

type IsAvaildMonth<S extends string> = NumberToArray<'12'> extends [...NumberToArray<GetNumber<S>>, ...infer R] ? true : false;

type GetResultDays<Month extends string> = GetNumber<Month> extends Day31 ? '31' : GetNumber<Month> extends Day30 ? '30' : GetNumber<Month> extends Day28 ? '28' : never;

type IsAvaildDay<Month extends string, S extends string> = S extends '00' ? false : NumberToArray<GetResultDays<Month>> extends [...NumberToArray<GetNumber<S>>, ...infer R] ? true : false;

type ValidDate<S extends string> = S extends `${infer One}${infer Two}${infer Three}${infer Four}`
  ? IsAvaildMonth<`${One}${Two}`> extends true
    ? IsAvaildDay<`${One}${Two}`, `${Three}${Four}`> extends true
      ? true
      : false
    : false
  : false;

Solution by CaoXueLiang #19515

type M1 = 01 | 03 | 05 | 07 | 08 | 10 | 12; type M2 = 04 | 06 | 09 | 11; type M3 = 02; type AddZero<T extends number> = ${T} extends ${infer K}${infer F} ? F extends `` ? 0${K} : ${T} : ${T}; type D1<T = 28, S extends 0[] = [0], R = never> = S['length'] extends T ? R | ${AddZero<S[length]>} : D1<T, [...S, 0], R | ${AddZero<S['length']>}>; type D2 = 29 | 30; type D3 = 31;

type ValidDate<T extends string> = T extends ${M1}${D1 | D2 | D3} ? true : T extends ${M2}${D1 | D2} ? true : T extends ${M3}${D1} ? true : false;

Solution by my5201314zwl #16935

// your answers

/**
 * StringToNumber<'01'> // 1
 * StringToNumber<'1'> // 1
 * SttringToNumber<''> // 0
 */
type StringToNumber<T extends string> = T extends `0${infer R extends number}`
  ? R
  : T extends `${infer R extends number}`
    ? R
    : 0

type PlusOne<T extends number, R extends 0[] = []> = R['length'] extends T
  ? [0, ...R]['length']
  : PlusOne<T, [0, ...R]>

/**
 * GetDateAndMonth<'0'> // [0, 0]
 * GetDateAndMonth<'0123'> // [1, 23]
 * GetDateAndMonth<'01234'> // [1, 234]
 */
type GetDateAndMonth<T extends string, C extends number = 0, Date extends string = '', Month extends string = ''> = C extends 2
  ? [StringToNumber<Date>, StringToNumber<T>]
  : T extends `${infer F}${infer R}`
    ? GetDateAndMonth<R, PlusOne<C>, `${Date}${F}`>
    : [StringToNumber<Date>, StringToNumber<Month>]

type NumberToTuple<T extends number, Res extends 0[] = []> = Res['length'] extends T
  ? Res
  : NumberToTuple<T, [...Res, 0]>
  
type MinusOne<T extends number, Res extends 0[] = NumberToTuple<T>> = Res extends [infer F, ...infer R]
  ? R['length']
  : never
  
type GT<T extends number, U extends number> = T extends U
  ? true
  : T extends 0
    ? false
    : GT<MinusOne<T>, U>

/**
 * GreaterThan<1, 2> // false
 * GreaterThan<2, 2> // false
 * GreaterThan<3, 2> // true
 */
type GreaterThan<T extends number, U extends number> = Equal<T, U> extends true
  ? false
  : GT<T, U>

/**
 * InRange<2, 1, 2> // false
 * InRange<2, 1, 3> // true
 */
type InRange<A extends number, F extends number, R extends number> = GreaterThan<A, F> extends true
  ? GreaterThan<R, A> extends true
    ? true
    : false
  : false

type ValidDate<T extends string, A extends [number, number] = GetDateAndMonth<T>> = InRange<A[0], 0, 13> extends true
  ? A[0] extends 2
    ? InRange<A[1], 0, 29> extends true
      ? true
      : false
    : InRange<A[1], 0, 32> extends true
      ? true
      : false
  : false

Solution by humandetail #16511

// your answers
type DaysInMonth =
{
  '01':31,
  '02':28,
  '03':31,
  '04':30,
  '05':31,
  '06':30,
  '07':31,
  '08':31,
  '09':30,
  '10':31,
  '11':30,
  '12':31,
} ;

type Dictionary =
{
  '0':[],
  '1':[0],
  '2':[0,0],
  '3':[0,0,0],
  '4':[0,0,0,0],
  '5':[0,0,0,0,0],
  '6':[0,0,0,0,0,0],
  '7':[0,0,0,0,0,0,0],
  '8':[0,0,0,0,0,0,0,0],
  '9':[0,0,0,0,0,0,0,0,0],  
} ;

type _10 =[0,0,0,0,0,0,0,0,0,0] ;

type Multyply<
    A extends readonly any[],
    B extends readonly any[]> =
    A extends [infer R,...infer U]
    ? [...B,...Multyply<U,B>]
    : []  ;

type ReverseString<
    S extends string> =
    S extends `${infer R extends string}${infer U extends string}`
    ? `${ReverseString<U>}${R}`
    : ''  ;

type ReverseStringToTuple<
    S extends string> =
    S extends `${infer R extends keyof Dictionary}${infer U extends string}`
    ? [...Dictionary[R],...Multyply<_10,ReverseStringToTuple<U>>]
    : []  ;

type ValidDate<
    S extends string> = 
    S extends `${infer R}${infer U}${infer V}`
    ? `${R}${U}` extends keyof DaysInMonth
    ? ReverseStringToTuple<ReverseString<V>> extends []
    ? false
    : ReverseStringToTuple<ReverseString<`${DaysInMonth[`${R}${U}`]}`>> extends
    [...ReverseStringToTuple<ReverseString<V>>,...0[]]
    ? true
    : false 
    : false
    : false ;

Solution by justBadProgrammer #16071

// your answers
type PlusOne<T extends number, Res extends 1[] = []> = Res['length'] extends T ? [...Res, 1]['length'] : PlusOne<T, [...Res, 1]>
type Rang<T extends number, Res extends any[] = ['00'], Flag extends boolean = Res['length'] extends 10 ? false : true> = 
  Res['length'] extends PlusOne<T> 
    ? Exclude<Res[number], '00'>
    : Flag extends true 
      ? Rang<T, [...Res, `0${Res['length']}`]>
      : Rang<T, [...Res, `${Res['length']}`], Flag>
type Date = {
  '01': Rang<31>,
  '02': Rang<28>,
  '03': Rang<31>,
  '04': Rang<30>,
  '05': Rang<31>,
  '06': Rang<30>,
  '07': Rang<31>,
  '08': Rang<31>,
  '09': Rang<30>,
  '10': Rang<31>,
  '11': Rang<30>,
  '12': Rang<31>,
}
type ValidDate<T extends string> = keyof {
  [
    P in keyof Date as 
      T extends `${P}${infer Rest}` 
        ? Rest extends Date[P] 
          ? P 
          : never 
        : never
  ]: any
} extends never ? false : true

Solution by Sliect #14667

Other solutions mainly use union types of string literals, I find it feasible to solve the problem without them:

type ValidDate<T extends string> = T extends `${infer M1}${infer M2}${infer D1}${infer D2}${infer Rest}` ? Rest extends '' ?
  `${M1}${M2}` extends keyof MonthDays ?
    `${D1}${D2}` extends '00' ? false :
      InRange<MonthDays[`${M1}${M2}`], `${D1}${D2}`>
  : false
: false : false;

type MonthDays = {
  '01': '31',
  '02': '28',
  '03': '31',
  '04': '30',
  '05': '31',
  '06': '30',
  '07': '31',
  '08': '31',
  '09': '30',
  '10': '31',
  '11': '30',
  '12': '31',
}

type GreaterMap = {
  '0': [],
  '1': ['0'],
  '2': ['1', '0'],
  '3': ['2', '1', '0'],
  '4': ['3', '2', '1', '0'],
  '5': ['4', '3', '2', '1', '0'],
  '6': ['5', '4', '3', '2', '1', '0'],
  '7': ['6', '5', '4', '3', '2', '1', '0'],
  '8': ['7', '6', '5', '4', '3', '2', '1', '0'],
  '9': ['8', '7', '6', '5', '4', '3', '2', '1', '0'],
}

type Greater<A extends string, B extends string> = A extends keyof GreaterMap ? Contains<B, GreaterMap[A]> : never;
type Contains<B extends string, ARR extends any[]> = ARR extends [infer Head, ...infer Rest] ?
  Eq<B, Head> extends true ? true : Contains<B, Rest>
: false;
type GreaterOrEq<A extends string, B extends string> = Greater<A, B> extends true ? true : Eq<A, B>;
type Eq<A extends any, B extends any> = A extends B ? B extends A ? true : false : false;

type InRange<R extends string, T extends string> = R extends `${infer R1}${infer R2}` ?
  T extends `${infer T1}${infer T2}` ?
    Greater<R1, T1> extends true ? true : Eq<R1, T1> extends true ? GreaterOrEq<R2, T2> : false
  : never
: never;

Solution by darkyzhou #11787

type ValidDate<T extends string> = T extends M28 | M30 | M31 ? true : false

type _0_8 = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8
type _0_9 = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
type _1_9 = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
type D28 = `0${_1_9}` | `1${_0_9}` | `2${_0_8}`
type D30 = `0${_1_9}` | `1${_0_9}` | `2${_0_9}` | `30`
type D31 = `0${_1_9}` | `1${_0_9}` | `2${_0_9}` | `30` | `31`
type M31 = `${`01` | `03` | `05` | `07` | `08` | `10` | `12`}${D31}`
type M30 = `${`04` | `06` | `09` | `11`}${D30}`
type M28 = `02${D28}`

Playground

Solution by teamchong #11748

09155-hard-validdate

9155 - ValidDate

🎥 Video Explanation

🔢 Code

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