Hi
I wounder if you have not done a mistake in how you use the RTC, the DS has a "funny" way to handle numbers. They are BCD coded and can not be written as Decimal numbers straight. You say you write I2CWrite PORTC.2,PORTC.3,$D0,$00,[0, 15, 15, 5, 1, 10, 9, 10]. Please look at the datasheet Timekeeper registers. If you want to load 15 minutes in the DS you have to write it as 21DEC. Why? Becasue the low 4 bits are minutes and the higher 3 bits are 10 minutes so.... Low bits should be 5 (%0101) and then we need 1 ten-minutes thus getting %00010101

Minutes and Hours are kind of straight forward BCD but the Hours register you have to setup for 12pm or 24 hours function and on top of this use BDC codes.

You can not use almost any register from the DS without convering it from BCD code and you get 90 and the strange jumps becasue of this.