Setting time into a microcomputer

When the program in a real-time computer needs to know the time of day and the date, there are two ways this can be implemented. One way is to have a software clock: every, say, 100 ms the main program is interrupted to go into a subroutine which updates the time. The other technique is to use a hardware clock, made up from a counter that divides by 60 for seconds and so on.

In the software case, when first switching on, the time of day and date is usually entered via the keyboard. In the hardware case, when first switching on, the time of day and date is set in via switches.