Together with hh:mm(:ss) for times and +hh:mm for timezones. Don’t make me deal with that 12am/pm bullshit that doesn’t make any sense, and don’t make make me look up just what the time difference is between CEST and IST. Just give me the offsets +02:00 and +05:30, and I can calculate that my local time of 06:55+03:30=10:25 in India.

You never know when something might need to become a filename, so you might as well just use ISO 8601 for everything.