Most are not. Those that are are not represented in any standard way. In all of the various date formats I have looked at, each one has certain flaws. ISO requires that you know a week number, and nobody thinks like that. Expressing the date and the time as separate fields is too much work to convert back to something useful.
Julian dates are amazingly non-intuitive.
And then I encountered ICal format. And, it is sortable. Here's my proposal, and I really want your comments. It will know about the ICal format, and will probably be able to convert to epoch time, but I'm not sure if that is desirable or not. It will have an ical method to return a correctly-formatted ical string.