Re: Need localization-proof method of transporting dates.
Re: Need localization-proof method of transporting dates.
- Subject: Re: Need localization-proof method of transporting dates.
- From: Marcel Weiher <email@hidden>
- Date: Sat, 4 Apr 2009 13:36:12 -0700
On Apr 4, 2009, at 5:01 , Graham Cox wrote:
One problem I've had occasional reports of is that the expiry is
prematurely detected, and it seems to be on systems with system
language set other than English. I need to store and check the dates
in a way that is not sensitive to this. I thought I was, yet the
reports persist.
Dates are tricky. One issue is that what's consistent in a machine
sense is often at odds with consistent from the user's point of view,
and that various layers of interpretation get you a bit of a muddle
(classic case is calendar programs treating dates as absolute times,
so when you schedule appointments in a different time-zone for a trip
and then switch to that time-zone during the trip, all your
appointments shift).
The muddle problem can be largely eliminated by storing a fairly
unambiguous value, such as seconds since 1970 (or 2000 see
NSTimeInterval), preferably as a 64 bit value (NSTimeInterval's double
should also work for most applications..).
Once you've left the muddle behind, you are left with
interpretations. What time-zone is the value interpreted in? GMT?
The user's time-zone? When are offsets applied?
If you are archiving the date, my guess is that it is being archived
with your local time-zone, and then compared to a date that is in your
client's time-zone.
The solution I have is to store the date year/month/day, completely
independent of NSDate. I then construct the expiry NSDate locally, so
it will be interpreted in the same context as the current date that
"[NSDate date]" delivers.
I also tend to not say when exactly the software expires... :-)
Hope that helps,
Marcel
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