Keeping accurate time

Hey guys,
So I'm curious how to create a timer with the correct value to keep
track of seconds.

When my device starts up, it receives the time via UART communication
from another device.  Instead of having it ask every second what time
it is, I'd like the msp430 to keep track of the time from that point
on, and possibly ask for the time only every minute. (just to be safe)

I'm using a msp430f2272, so i've got two timers, I'm just not sure if
i'll need an external crystal, or if i could use the internal ones to
keep track of seconds or milliseconds.


Thanks.
Jonathan