Date/time should always be saved in UTC in the database and used in UTC internally in the system and only displayed to the user in its favorite timezone and time-format. (Also, the PC Real Time Clock should run in UT)
It freaks me out when I see an Internet site that still shows date/time of the content in the machine that hosts that content in its local time instead of UTC.
I am a big supporter of ISO 8601 and I use it everywhere.
You should be using ISO 8601 and UTC in case of any doubt.