In the Gregorian calendar, which is the calendar used by most modern countries, the following rules decides which years are leap years:
Every year divisible by 4 is a leap year.
But every year divisible by 100 is NOT a leap year
Unless the year is also divisible by 400, then it is still a leap year.
This means that year 1800, 1900, 2100, 2200, 2300 and 2500 are NOT leap years, while year 2000 and 2400 are leap years.
This actually means year 2000 is kind of special, as it is the first time the third rule is used in many parts of the world.

http://www.timeanddate.com/date/leapyear.html

Basically, the answer is that for purposes of calculating leap-day, it is the same in either century. Ie, 1904 and 2004 (etc.) are the same. So software century works. (I should have reviewed leap year before I started this thread, but maybe it will help someone else.) The RTC doesn't need to know or care which cenury. I plan to check the DS1302 for 1900 and 2000 just for fun, but I don't plan any trips back in time....

Steve McLaughlin