From the OpenBSD at(1) man page:
[at(1)] accepts times of the form HHMM or HH:MM to run a job at a specific time of day. (If that time is already past, the next day is assumed.) You may also specify midnight, noon, or teatime (4pm) and you can have a time-of-day suffixed with “AM” or “PM” for running in the morning or the evening.
I just think being able to specify “at teatime ….” is cute.