RISC OS actually has two clocks that keep track of time;1) the hardware clock and 2) the software clock.
Both clocks are timed independently and can diverge by a few seconds per day. The clocks are only re-synchronised when the computer system has been hardware reset.
Applications that require the use of time and date settings should always use the time provided by the software clock. The software clock measures time in centi-seconds.
Note: Times are represented using 32-bits, and it should be remembered that the number used can go negative. i.e when bit 31 is becomes 1. So when comparing times, the expression
(new_time – old_time) > 100
is better than
new_time > old_time + 100