The real time System (RTS) :

Calling the clock real-time is somewhat of a misnomer because it only shows the time setting it has been given. The RTC is the other half of chip that has CMOS memory and may be thought of as a set of counters.

The first one counts from 0 to 9 and then tells to the next counter (the 10's place counter) to count once. The first counter then begin counting again and counts from 0 to 9 and again tell the next higher counter (the 10's counter) to set its counter up one more (now at 2), and so on.

Now the 10's place counter (which is counting 10's of seconds) only counts from 0 to 5 and then it tells the minute counter to do its increment, which will then start its drive from 0 to 9 and so on.

Of course the next counter after minute counter is minute 10's place counter, which also counts from limit 0 to 5 and then tells the hours counter to count once etc, and so on.

The procedure goes on through the hour 10's place as it counts from 0 to 2, the day counter-which enter from 1 to 30, or  1 to 31, or sometimes  1 to 28 or might be 29 depending on what the rules for which month has how many days and which years have 29 day Februarys. The month counter proceeds to enter from 1 to 12, and of course then the year counter starts its trip with the good old 0 to 9, and in last we have year 10's counter again with values going from 0 to 9.