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From: psfales@ihlpl.UUCP (Peter Fales)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Date does not change when PC is left on
Message-ID: <1538@ihlpl.UUCP>
Date: Wed, 3-Dec-86 17:40:04 EST
Article-I.D.: ihlpl.1538
Posted: Wed Dec  3 17:40:04 1986
Date-Received: Thu, 4-Dec-86 00:23:50 EST
References: <289@rocky2.UUCP>
Organization: AT&T Bell Labs, Naperville, IL
Lines: 22

> When we leave our PCs on overnight, the date does not always
> change, although the time is kept current.  I have not
> figured out how to duplicate so I don't know exactly why it
> happens only sometimes.

Just a guess, but does it only happen when the PC is not used for
two days (i.e. over a weekend)?

This may not be quite correct, because I don't remember all the details,
but the basic idea is this:  The way DOS keeps track of time
is to increment a counter each time the timer interrupt comes in (18
times per second).  Then the DOS routines convert this to the
actual date and time.  When the timer interrupt notices that a midnight
has been crossed it simply sets a flag - it is the responsibility
of the next call to the DOS time function to notice this flag and
increment the date.  However, if the DOS function is not called for
a whole day (i.e. the time crosses two midnights) the flag will
be set twice but the only noticed (and cleared) once, resulting in
a date that is one day behind.

Peter Fales
ihnp4!ihlpl!psfales