> Use 24 hour time internally.  Whether you then display 24 hour or
> 12 hour time with AM/PM is only a matter of the display logic

That's what I do with my clocks. Even using minutes for long-range
(years) calendar functions such as alarms and intervals. Internally only
seconds and minutes, and the display is updated after division by 1440
for days and a look-up table for day name/date/month. The maths and
manipulations are so much easier using just one basic unit

The complications of hours, 12/24, days, weeks, and years need only
be attended to for display and are a handicap for working out how
many minutes are between 10:31am 15th April and 11:07pm February
4th and when to set off regular alarms in that interval
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