Hello Robert,

I assumed you have read the Application data sheet but in case you have not you can find it at this web site http://www.spezial.cz/pdf/rtc-72421.pdf.

Section 4 touches on how to handle your problem. It would appear the signal control select 1 (CS1) can be used to control the timing when power up is occurring. It basically won't let anything change while power up is occurring. After power has stabilized then it is taken out of the picture.

You also mentioned only the hour or month is what get changed. What about the minutes and seconds. They are OK? Since they survive I would think you also might have a software problem. It wouldn't hurt to look at that as a potential problem.

I would bet control select 1 signal (CS1) would be your best bet

Good luck.

Regards,

Charles Bannister