On 16 Apr, 11:11 pm, f...@fuhm.net wrote:
On Apr 16, 2009, at 5:47 PM, Antoine Pitrou wrote:
It's a human-interface operation, and as such, everyone (ahem) "knows
what it means" to say "2 months from now", but the details don't
usually have to be thought about too much. Of course when you have a
computer program, you actually need to tell it what you really mean.
I do a fair amount of date calculating, and use two different kinds of
"add-month":
Option 1)
Add n to the month number, truncate day number to fit the month you
end up in.
Option 2)
As above, but with the additional caveat that if the original date is
the last day of its month, the new day should also be the last day of
the new month. That is:
April 30th + 1 month = May 31st, instead of May 30th.
They're both useful behaviors, in different circumstances.
I don't have a third option, but something that would be useful to
mention in the documentation for "monthdelta": frequently users will
want a recurring "monthly" event. It's important to note that you need
to keep your original date around if you want these rules to be
consistently applied. For example, if you have a monthly billing cycle
that starts on May 31, you need to keep the original May 31 around to
add monthdelta(X) if you want it to be May 31 when it rolls around next
year; otherwise every time February rolls around all of your end-of-
month dates get clamped to the 28th of every month. (Unless you're
following James's option 2, of course, in which case things which are
normally on the 28th will get clamped to the 31st of following months.)
My experience with month-calculating software suggests that this is
something very easy to screw up.
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