Press H and a clock slides down from the top of the screen with the day and time in your world. Press it again for your real date and time, again for both, and once more to put it away.

These are in the assortedtime-client config file. They only change your own game, so everyone can set their own.
time.twentyFourHour uses a 24 hour clock. On by default. Turn it off for AM and PM.Time also comes in Assorted Util with the other five Util mods, if you'd rather have them all in one download.
The full guide is at assortedmods.com. It's also in the game, in the Instruction Manual from Assorted Lib.
Found a bug? Report it on GitHub.