TypeScript – How to accurately type dates in ISO 8601 format

TS - More precise return type method Date#toISOString.d.ts

// In TS, interfaces are "open" and can be extended
interface Date {
  /**
   * Give a more precise return type to the method `toISOString()`:
   * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toISOString
   */
  toISOString(): TDateISO;
}

type TYear         = `${number}${number}${number}${number}`;
type TMonth        = `${number}${number}`;
type TDay          = `${number}${number}`;
type THours        = `${number}${number}`;
type TMinutes      = `${number}${number}`;
type TSeconds      = `${number}${number}`;
type TMilliseconds = `${number}${number}${number}`;

/**
 * Represent a string like `2021-01-08`
 */
type TDateISODate = `${TYear}-${TMonth}-${TDay}`;

/**
 * Represent a string like `14:42:34.678`
 */
type TDateISOTime = `${THours}:${TMinutes}:${TSeconds}.${TMilliseconds}`;

/**
 * Represent a string like `2021-01-08T14:42:34.678Z` (format: ISO 8601).
 *
 * It is not possible to type more precisely (list every possible values for months, hours etc) as
 * it would result in a warning from TypeScript:
 *   "Expression produces a union type that is too complex to represent. ts(2590)
 */
type TDateISO = `${TDateISODate}T${TDateISOTime}Z`;

Here's what I've been using:

type oneToNine = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;
type d = oneToNine | 0;
type YYYY = `19${d}${d}` | `20${d}${d}`;
type MM = `0${oneToNine}` | `1${0 | 1 | 2}`;
type DD = `${0}${oneToNine}` | `${1 | 2}${d}` | `3${0 | 1}`;
type DateYMString = `${YYYY}-${MM}`;
type THours = `${number}${number}`;
type TMinutes = `${number}${number}`;
type TSeconds = `${number}${number}`;
type TMilliseconds = `${number}${number}${number}`;

// type: DateYMD ex. "2021-01-01"
export type DateYMD = `${DateYMString}-${DD}`;

/**
 * type: TDateISOTime ex. "14:42:34.678"
 */
type TDateISOTime = `${THours}:${TMinutes}:${TSeconds}.${TMilliseconds}`;

/**
 * type: DateYMDZoneString ex. "2021-01-01T14:42:34.678Z"
 */
export type ISO8601 = `${DateYMD}T${TDateISOTime}Z`;

My aim was not to make this perfect but to help point people in the right direction.