Branded strings that represent dates in TypeScript.

dateString.ts

enum DateStringBrand {}

export type DateString = string & DateStringBrand;

function of(date: string | number | Date): DateString {
    // This branching needed to satisfy TypeScript.
    const dateObject =
        typeof date === "string"
            ? new Date(date)
            : typeof date === "number" ? new Date(date) : new Date(date);
    if (isNaN(+dateObject)) {
        throw new Error(`Cannot create DateString from "${date}".`);
    } else {
        return dateObject.toISOString() as DateString;
    }
}

function now(): DateString {
    return of(Date.now());
}

export const DateString = { of, now };