Absence creation dialog doesn't handle min/max date times correctly

two issues:

• start date time is limited to end date time – and the latter is automatically set to the end of the current day. thus, it has to be changed first in order to allow a later start date time. probably, better error messages or something like that (along the lines of "the start date must be earlier than the end date") are enough to "fix" that
• manually entering the same dates (with different times) results in the start date being considered as being too early