Examples of ISO format: YYYY-MM-DD or YYYY-MM-DDTHH:MM:SS.
But however, just using the "ISO format" doesn't work reliably by itself. String are sometimes parsed as UTC and sometimes as localtime. The best practice should always be to store dates as UTC and make computations as UTC.
To parse a date as UTC, append a Z - e.g.: new Date('2011-04-11T10:20:30Z').
To display a date in UTC, use .toUTCString(),
To display a date in the user's local time, use .toString().