[time-nuts] How best to compute local time from GPS
Mike S
mikes at flatsurface.com
Tue Mar 25 08:54:55 EDT 2008
At 11:07 PM 3/24/2008, David Forbes wrote...
>2. If I have to store the time zone from the user's input, are the
>DST calculations reasonably straightforward these days?
>
>3. What weird time zone operations should it support, such as 15
>minute local offsets or oddball DST dates?
>
>4. In general, is it better to let the user turn DST on and off or
>try to do it automatically? (I live in Arizona, which doesn't worship
>DST, so I have no experience in this matter.)
There is a well accepted method of specifying time zone offset and
summer time adjustments. It is much simpler than trying to parse an
Olson timezone table (which has historical information to allow date
and calculations, something not needed for a clock). It allows for the
full range of rules, and you don't have to worry about updating a table
should the rules change. This is called a IEEE 1003.1 POSIX timezone
literal. A good description is contained in
http://tools.ietf.org/html/draft-ietf-dhc-dhcpv6-opt-tz-00
To specify the timezone for the eastern US, including current daylight
saving time rules, one would use
TZ="EST5EDT4,M3.2.0/02:00:00,M11.1.0/02:00:00"
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