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From: LEICHTER-JERRY@YALE.ARPA.UUCP
Newsgroups: mod.computers.vax
Subject: Re: date/time arithmetic in vms
Message-ID: <8607031245.AA07794@ucbvax.Berkeley.EDU>
Date: Thu, 3-Jul-86 08:47:09 EDT
Article-I.D.: ucbvax.8607031245.AA07794
Posted: Thu Jul  3 08:47:09 1986
Date-Received: Thu, 3-Jul-86 23:37:55 EDT
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Reply-To: <LEICHTER-JERRY@YALE.ARPA>
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    If I want to compute a date (VAX quadword date form) that is 1 month from
    today - what is the best way?
The quadword format is the one to work with; since it is a simple binary
number - yes, I know it's a quadword - you can do arithmetic on it:

	a)  Use $GETTIM to get the current time into quadword NOW.
	b)  Now you need, in quadword DELTA, a delta time that represents
		"one month".  Unfortunately, there is no notation for "plus
		one month" - only for "plus some number of days".  So you'll
		need to compute the number of days in the current month ("30
		days hath September, all the rest I don't remember", to quote
		Martin Minow) - call it dd - and convert the delta time
		"dd 00:00:00.00" into DELTA, again with $BINTIM.
	c)  The value you want is NOW - DELTA.  (It's "-" rather than "+"
		because delta times in quadword format are stored negated.)

That leaves the problem of doing quadword arithmetic, something no HLL does
directly.  However, there is a library procedure - LIB$SUBX - to do exactly
that.  The quadword values themselves your HLL program will consider to be
arrays of 2 longwords.

There is an alternative approach:  Rather than using $NUMTIM, use $ASCTIM.
The ASCII string you get back isn't nearly as convenient for manipulation as
the array $NUMTIM gives you, but you CAN convert it back with $BINTIM.

							-- Jerry
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