new date conversion bug?

[Sarah Reichelt]

On Feb 1, 2008 6:52 AM, Devin Asay <devin_asay at byu.edu> 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 would assume that daylight savings changes for you between March 7 &
March 21, so that explains the hours difference.

It has long been a source of annoyance to me that "the seconds" is
altered to allow for time zones and daylight savings, so that the same
number will convert to a different time & date on different systems
and depending on the time of year it refers to. Effectively, this
means that you cannot really use "the seconds" for such date & time
operations.

How about this sort of thing (again in pseudo code):

get the date of the first event
convert it to dateItems
add 14 to item 3
convert back to date (which will work even if you have something that
looks weird in date items e.g. 2008,3,42,....)

Cheers,
Sarah