Ian Moote wrote to JONATHAN DE BOYNE POLLARD on 12-01-1999

IM> DOS uses internal counters. It only reads the RTC when it boots.
IM> It only  writes the RTC when you set the system time. 

For the sake of completeness, the RTC is also sync'd by a
read/write/read anytime the set clock API's are called.  There are also
several int 21 calls that force a read of the RTC and reset the
internal timer to reflect the number of (computed) clock ticks since
midnight.  Some of these are in fact the source of the DOS problem of
not incrementing the day if you hit the clock routines just right
during the midnight roll-over.  To this day there are problems with DOS
sessions playing games with time set/read at mifnight under OS/2 (and
NT for that matter).

Will Honea 
--- Maximus/2 2.02