new date conversion bug?

[J. Landman Gay]

Devin Asay wrote:
> I am working on a scheduling program that includes the ability to 
> schedule an event every other week over a specific time period. To 
> determine whether the event should occur during any given week during 
> the time period I do some simple date math to figure out whether a 
> multiple of 14 days have passed since the first  occurrence of the 
> event. Let's say I've scheduled an event for every other week on Friday, 
> beginning January 11, 2008 and ending April 11, 2008. For any given week 
> on my calendar I can calculate whether the event should show up using 
> this algorithm (pseudocode):
> 
> get the date of the first occurrence of the event and convert it to seconds
> get the date of that the event should fall on if this week falls in the 
> every-other-week pattern, convert it to seconds
> subtract date 2 from date 1
> divide the difference by (60*60*24*14) --the number of seconds in 14 days
> if the difference divides evenly (modulo division = 0) then the event 
> should occur in the week in question
> 
> This works fine for a few weeks, but then inexplicably fails. For 
> example, Jan. 11, Jan. 25, Feb. 8, Feb. 22, and March 7 all show the 
> event properly. But anything after that fails; i.e., March 21, April 4.

I'm not sure why it would fail; that one hour difference is suspiciously 
like a time zone error or a daylight savings time error (which would 
still be a bug, if that's what's happening.)

But changing the calculation to use dateitems seems to work okay (and 
avoids all those inscrutable big numbers):

   put "1/11/08" into tDate
   put "4/15/08" into tStopDate
   convert tStopDate to seconds
   repeat
     convert tDate to dateitems
     add 14 to item 3 of tDate
     convert tDate to seconds
     if tDate > tStopDate then exit repeat
     convert tDate to short date -- if you need that
     put cr & tDate after fld 1 -- or fill your calendar field here
   end repeat

-- 
Jacqueline Landman Gay         |     jacque at hyperactivesw.com
HyperActive Software           |     http://www.hyperactivesw.com