javascript to check if a string is a valid YYYY-MM-DD-style date or datetime. This is more permissive than ISO 8601 but less permissive than the Date() constructor.

is_valid_yyyy_mm_dd_date.js

/*
 * returns true/false on "is this string a valid date/datetime string"
 *
 * any credit for coolness goes to https://stackoverflow.com/a/44198641,
 * and any blame for wrongness goes to me. :)
 *
 */
const isValidDate = dateString => {
  let date = new Date(dateString);                              // we're gonna use both :0
  return (
    date &&                                                     // make sure it's non-falsey,
    /[0-9]{4}-[0-9]{2}-[0-9]{2}/.test(dateString) &&            // roughly YYYY-MM-DD.* to stop e.g. new Date('0'),
    Object.prototype.toString.call(date) === '[object Date]' && // is actually a date object,
    !isNaN(date)                                                // and is actually a *valid* date object.
  );
};

e.g.

isValidDate(Date(0))
// -> false
isValidDate(null);
// -> false
isValidDate("hello");
// -> false
isValidDate("2012");
// -> false
isValidDate("2");
// -> false
isValidDate("2012-01");
// -> false
isValidDate("2012-01-1");
// -> false
isValidDate("2012-01-01");
// -> true
isValidDate("2012-01-01T00:00");
// -> true
isValidDate("2012-01-01 00:00");
// -> true
isValidDate("2012-01-01T00:00:00.0");
// -> true
isValidDate("2012-01-01 00:00:00.0");
// -> true
isValidDate("2012-01-01 00:00:00.1");
// -> true
isValidDate("2012-01-01T00:00:0");
// -> false
isValidDate("2012-01-01 ok:00:00");
// -> false
isValidDate("2012-01-01 00:00:00Z");
// -> true
isValidDate("2012-01-01 00:00:00 GMT");
// -> true
isValidDate("2012-01-01 GMT");
// -> true
isValidDate("2012-01-01 +0500");
// -> false
isValidDate(new Date('2012-01-01 00:00:00').toGMTString()) // doesn't work with non-YYYY-MM-DD strings
// -> false