JavaScript Date-only Format Guaranteed To Be Interpreted As Local Time In Modern Browsers

I am looking for a date format (only year, month and day, no time component), that will be guaranteed to be interpreted as local time, in all reasonably modern browsers, when passe

Solution 1:

I am looking for a date format (only year, month and day, no time component), that will be guaranteed to be interpreted as local time, in all reasonably modern browsers, when passed to the Date() constructor in JavaScript.

There isn't one. There are a number of formats that, in practice, are reliably parsed by all browsers in use however none of them are required by ECMA-262 to be supported by ECMAScript implementations.

There is one format, a version of ISO 8601, that is specified in ES5 and ECMAScript 2015 however dates without a time zone are to be treated as UTC in ES5 and "local" in ECMAScript 2015.

But that was seen by implementers as "breaking the web", so while some browsers implemented that for a while, they have now reverted to ES5 behaviour and treat a missing timezone as UTC.

A small change in format such as changing the separator from "-" to "/" causes a reversion to implementation dependent behaviour and may cause the date to be treated as UTC or local (though most seem to treat 2015/12/17 as local).

So if you want a guarantee, there isn't one. However, if you are prepared to write a 2 line function, you can parse an ISO 8601 like string reliably based on whatever time zone you like. If you want it to be treated as local (i.e. set to 00:00:00 and offset based on system settings), then:

function parseISOLike(s) {
    var b = s.split(/\D/);
    return new Date(b[0], b[1]-1, b[2])
}

document.write(parseISOLike('2015-12-17'));

will do the job (without checking for valid values) and allow the separator to be any non–digit, so 2015/12/17, 2015-12-17 and 2015.12.17 will all be correctly parsed.

Edit

Parts of the above have been edited thanks to input from Matt Johnson.