Just to clear up confusion.
I don't need to display the time, Although I need to make accurate account of it.
I just need to sound alarm in preset schedule for a week. The schedule will have resolution of only minutes (or even five minutes), so no worries if I can't sound alarm at xx:xx:51 but only xx:xx:52. I just count how many 2-seconds have passed.
I am afraid to write to Timer1 registers, because I have read somewhere (perhaps in microchip forum), that It may introduce some random error to the Timer1 counting process.
But anyways, I don't need that.
Do you have a good reference on how I can apply calibration to my RTC. The first thought coming to my mind, is to let it run for 1 week, see how much time it looses or gains, and then write a code, that freezes or jumps a few seconds at each weekend.