PHP timezones explained

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PHP timezones example
written by Simon Hutchison - http://www.econfirm.com.au/

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date_default_timezone_set("Australia/Sydney")
Sun, 02 Mar 2008 14:57:11 +1100
date_default_timezone_set("Australia/Perth")
Sun, 02 Mar 2008 12:57:11 +0900
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The comments in this example assumes server is Sydney time.
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Demonstrating that timezone is not important for mktime(), date("U") and time().
Get GMT 0. Uses the server time to adjust to GMT 0 so need to ensure server time is correct for these functions to be accurate.

date_default_timezone_set("Australia/Perth")
mktime() : 1204430231
date("u") : 1204430231
time() : 1204430231

date_default_timezone_set("Australia/Sydney")
mktime() : 1204430231
date("u") : 1204430231
time() : 1204430231
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Now that we know this we can use these functions to compare to other GMT 0 adjusted times.

Demonstrating that timezone is critical for date("date format string") and strtotime("date string")
because it takes a user string from anywhere in the world!!


date_default_timezone_set("Australia/Sydney")
The next values of time() and strtotime will be very close (within a second) because both the default timezone and the server time is Sydney.
strtotime(): 1204426991
time() : 1204430231 just after the strtotime converted date/time
date("r") : Sun, 02 Mar 2008 14:57:11 +1100

date_default_timezone_set("Australia/Perth")
The next values of time() and strtotime will be about two hours apart because the default timezone is Perth and the server time is Sydney.
strtotime(): 1204434191
time() : 1204430231 about two hours before the strtotime converted date/time for Perth time. (Remember the server is in Sydney)
date("r") : Sun, 02 Mar 2008 12:57:11 +0900

This means that mktime() can be used to compare any normalised (strtotime with timezone set) time
Normalise all incomming user dates using strtotime with date_default_timezone_set set correctly.

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