Timing delay in RL78/G22

I’m currently working on a project using the Renesas RL78/G22 microcontroller, where I need to implement the led is ON for 7 hours and led is OFF for 17 hours for 160 days. I am using rtc as subsystem clock(fsxr)-32.678 Khz external clock. When I trigger the switch the led will On for 7 hours (10:00:00) and off for 17 hrs(05:00:00) this is my condition but the problem is if i trigger the switch the led is on at 10:00:00 the led should off at 05:00:00 but the led is turning off at 04:59:00.If this happens continuosly at the 160 th day there will be 3 hours delay. How to solve this problem?  Can please someone help me out.