----- Original Message -----
From: "Niklas Karlsson" <niklas.karlssonn@xxxxxxxxx>
To: <php-general@xxxxxxxxxxxxx>
Sent: Tuesday, April 10, 2007 5:32 PM
Subject: RE: Dates and different time zones
Thanks for good answers.
Okay, so I should set the default time zone to central GMT time, and then
save GMT offset for every user. This sounds realistic, because I don't
think
that I need to correct the time for users that doesn't login.
Well, that was my whole point. If you store the GMT offsets for each user
at a certain time in the year, they will turn invalid when they switch from
daylight savings to no savings. Not all countries use daylight savings and
even those that do don't switch at the same time. Furthermore, northern and
southern hemispheres switch in opposite directions, if and when they do. If
your users have to log in, that is the perfect time to read their current
local time from the browser and keep it just for the session. Don't store
it permanently.
In PHP dates are stored as seconds from an arbitrary zero set at Jan 1st,
1970, so does MySql with date/time values though it shows them formatted but
you can use the UNIX_TIMESTAMP function to get the actual timestamp.
JavaScript uses milliseconds from the same base date. The rest is plain
arithmetic.
Satyam
So, if I now have the GMT offset for every user, how do I display the
right
date? Can someone please show some phpcode? I would be very grateful if
someone could do that.
Satyam wrote:
Store all of your dates as GMT. Perform all date based calculations
around GMT also, and then offset the values for localised display only.
This way you only need to store the GMT offsets for each user, i.e.
GMT+1 or GMT-8 when it comes to displaying the dates to them. The
trick is to use a constant base date for all data, and only being the
user timezones into play when needed.
Actually, I find that it is better not to bother storing anything for
the user at all. At the first chance, get some JavaScript to read the
local time of the client machine and send it back to the server, either
with the login data, using some AJAX or along with any link the user
might click on the welcome screen, for example, the language choice.
Then use the offset from his local time to the server time for every
time information, substract it from any time information you read from
them, add it to anything you send them. This works whether the user is
registered or not, whether he/she travels or remains in the same time
zone and spares you the trouble of keeping your IP to country to
timezone table updated. It assumes that the user updates the time zone
on his/her machine and if he doesn't it means she doesn't care, so why
should you. (some travellers prefer to keep their portable machines set
to their home-base time zone)
Of cause the major fault with this is that it can only display the CURRENT
time offset. You *ALSO* need the users Daylight Saving Zone as well. This
has
been giving us great fun since the winter dates and times need a different
offset to the summer ones. Something that simplistic browser time offset
does
not supply. :(
The only way to get this working properly at present is to get the user to
set
their time/daylight settings in their profile, and then you can provide
the
correct offset for all days on a calendar. Remember that for users WITH a
daylight saving offset, one day each year has 23 hours and one 25 hours ;)
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