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On User-Defined Timezones in PHP - Page 2

by: David Whittington
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September 11, 2003

Why use Timestamps?
When we initially designed our portal, the choice was made to store time values in the database as timestamps. This means that as soon as a user enters a date, it is converted from a text string to a timestamp for internal manipulation and storage, and only converted back to human-readable form when being displayed back to the user. Using timestamps it is possible to easily manipulate the date with a combination of simple math and the powerful functions provided by PHP ('date' and 'mktime').
For example, finding midnight of any given day is a simple matter of applying this equation: $timestamp - ($timestamp % (24*60*60)). It is also handy to use the very flexible 'mktime' function. It allows the programmer to do strange things like input "January -1, 2000" and expect a timestamp representing "December 31, 1999." The 'date' function is also very powerful and allows for formatting of a date string from any timestamp. But, lurking behind their seemingly benign facade is a maelstrom of debugging horrors.

The Inadequacy of PHP Time Functions

PHP's 'mktime' and 'date' functions work well as a pair without the help of any other timestamp manipulation routines, but only if the application in which they are used is concerned solely with display and entry of time in the servers timezone. If an application needs to handle entry from a timezone other than that in which the server is located something more than 'mktime' and 'date' is required.
Two things are required to accomplish this: a location independent format for storing time in the database, and methods to translate to and from that format into the user's local time. Simply deciding to store all timestamps in the database as seconds since the epoch (12:00AM January 1st 1970) in GMT is sufficient for determining the location independent time format. However, writing methods to translate to and from that format is a bit more difficult. It is not a simple task to translate a value like "2:00AM August 18th 1982" into a single integer. There are all kinds of nasty issues involving leap years, daylight savings, and such that must be dealt with to get the timestamp value. Fortunately as we already mentioned PHP has a nice set of functions to do all that for you. However a very specific knowledge of what those functions do is necessary in order to use them to convert the timestamps they return into a location independent format.
Unfortunately to many people the PHP documentation will not be very helpful on this point. Although to an enlightened few who are already well versed in the wonders of *nix timestamps it may be clear from reading it what 'mktime' actually returns; for most of us the answer is not so obvious.

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