https://twitter.com/mkldny/status/1374360618950651909?s=21

datestring.ts

type Day28 = '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;
type Month = '01'|'02'|'03'|'04'|'05'|'06'|'07'|'08'|'09'|'10'|'11'|'12'
type Digit = 0|1|2|3|4|5|6|7|8|9;
type Year = `${19 | 20}${Digit}${Digit}`

type IsDivide4<S extends string | number> = `${S}` extends '0' | '4' | '8' ? true :
    `${S}` extends `${number | ''}${`${0|2|4|6|8}${0|4|8}`|`${1|3|5|7|9}${2|6}`}` ? true : false;
type IsLeapYear<S extends string | number> =
    `${S}` extends `${infer Hundreds}00` ?
        IsDivide4<Hundreds>:
        IsDivide4<S>;

type DateString = 
    |`${Year}-${'01'|'03'|'05'|'07'|'08'|'10'|'12'}-${Day28|29|30|31}`
    |`${Year}-${'04'|'06'|'09'|'11'}-${Day28|29|30}`
    |{[Y in Year]: `${Y}-02-${IsLeapYear<Y> extends true ? Day28 | 29 : Day28}`}[Year]

let dateStrings: DateString[] = [
    '1900-01-01',
    '1900-01-31',
    '1900-12-31',
    '1900-11-31',
    '1900-11-30',
    '1900-02-29',
    '1904-02-29',
    '1900-02-28',
    '1901-02-28',
    '1901-02-28',
    '1901-02-29',
    '2015-02-28',
    '2016-02-29',
    '2017-02-29',
    '2018-02-29',
    '2019-02-29',
    '2020-02-28',
];