There's a bunch of other idea that spring to mind. Is the unit will be battery powered or AC powered?

If it's AC powered, i would prefer to use the AC as timebase, use an internal timer interrupt to get the 1 Second or 1 minute. Once in the interrupt do the math and check for Alarm or Events.

BUT you can do it with the internal Tick FOSC/4, in this case use one of the lower Crystal frequency... let's say 4 MHZ => internal = 1MHZ => ticks every 1 uSec => internal timer interrupt every 1000 counts to have a time base of 1 Sec => you need a 16 bits Timer.

Wich PIC Marc will use ???

Process is almost the same than with an RTC but you'll have to deal with a bunch of more code line to take care of the days of week/month/year/Leap Year/hours/minutes... BLAH BLAH BLAH, tsé veux dire !!!

NOT impossible, a bit tricky, i agree.

What the unit will also do?