Lester Caine wrote:

> Leyne, Sean wrote:
>> Lester,
>>
>>>>> I agree but want to suggest that if it is necessary for a
>>>>> system to commonly convert datetime values between timezones
>>>>> that storing the values as UTC is the only real solution.
>>>> ...
>>>>
>>>> My point is that if you store a timestamp including it's
>>>> time-zone you can have your cake and eat it too!
>>> NO
>>>
>>> If you store a meeting as 9AM British Summer Time, and then the
>> meeting
>>> gets moved to the following week - GMT - The there is no way of
>> telling
>>> from the ISO8901 timezone stamp that you need also to adjust an hour!
>> Actually, you can.
>>
>> Moving the item is not a simple mathematical operation of adding 7 days
>> to the current value but also adjusting the timezone portion of the
>> date/time based on the UTC offset will be for the same **time of day**
>> for the new date.
>>
>> Consider:
>>
>> Event time = "2009-03-11T10:00-05:00"
>>
>> If this event is moved to March 18th, you would need to get the UTC
>> offset for 10:00am March 18 (March 18 is UTC-4, daylight saving).
>>
>> So, the new meeting date/time will become
>>
>> Event time = "2009-03-18T10:00-04:00"
>>
>>
>>> Saving ISO timestamps with a time is pointless unless you also know
>> the
>>> location that relates to that timestamp.
>> That really only matters if the daylight savings rules change and you
>> expect the meeting time to maintain the **time of day** context.
>
> Sean - you are missing the point! How do I know that -05:00 is in a
> daylight saving area? A large part of the world simple does not bother
> and some smaller areas have different rules. ISO timestamps simply
> sidestep the fact that their data MAY be wrong for 50% of the year.

Having written the longer reply to Geoff, I'll reword that ...
ISO timestamps simply sidestep the fact that you do not really know what
the local time was 50% of the year. The UTC time is not wrong, just
whether the offset is for a physical timezone, or a DST corrected one.
You can guess based on date but you can't rely on that.

--
Lester Caine - G8HFL
-----------------------------
Contact - http://lsces.co.uk/wiki/?page=contact
L.S.Caine Electronic Services - http://lsces.co.uk
EnquirySolve - http://enquirysolve.com/
Model Engineers Digital Workshop - http://medw.co.uk//
Firebird - http://www.firebirdsql.org/index.php