Calculating GMT offsets

Another piece of information that's sometimes useful when working with time zones is the GMT offset -- that is, the difference between the specified time zone and standard GMT. The PEAR Date class lets you get this information easily, via its getRawOffset() method. Here's an example:

<?php
// include class
include ("Date.php");

// initialize object
$d = new Date("2006-06-21 10:36:27");

// set local time zone
$d->setTZByID("PST");

// get raw offset from GMT, in msec
echo $d->tz->getRawOffset();
?>

Here, the getRawOffset() method calculates the time difference between the local time and GMT. Here's the output:

-28800000

Note that this offset value is expressed in milliseconds, so you will need to divide it by 3600000 (the number of milliseconds in one hour) to calculate the time zone difference in hours.

Tip: You can use the inDaylightTime() method to see if the destination is currently observing daylight savings time. Look in the class documentation for details on this method.

Adding and subtracting timespans

The Date class also lets you perform sophisticated date arithmetic on temporal values, adding or subtracting durations to a date/time value. These durations (or timespans) are expressed as a string containing day, hour, minute and/or second components.

<?php
// include class
include ("Date.php");

// initialize object
$d = new Date("2006-06-21 15:45:27");

// add 01:20 to it
$d->addSpan(new Date_Span("0,1,20,0"));

// retrieve date as formatted string
echo $d->format("%A, %d %B %Y %T");
?>

In this case, I've added an hour and twenty minutes to the initial timestamp, by calling the Date class' addSpan() method and supplying it with a Date_Span() object initialised to that duration. The output is fairly easy to guess:

Wednesday, 21 June 2006 17:05:27

Just as you can add timespans, so too can you subtract them. That, in fact, is the purpose of the subtractSpan() method, which is illustrated below.

<?php
// include class
include ("Date.php");

// initialize object
$d = new Date("2006-06-21 15:45:27");

// add 01:20 to it
$d->addSpan(new Date_Span("0,1,20,0"));

// subtract 00:05 from it
$d->subtractSpan(new Date_Span("0,0,5,0"));

// retrieve date as formatted string
echo $d->format("%A, %d %B %Y %T");
?>

Here, I've first added an hour and twenty minutes, and then subtracted a further five minutes. The net effect is an addition of an hour and fifteen minutes, and the output reflects this:

Wednesday, 21 June 2006 17:00:27

As the examples above illustrate, the PEAR Date class provides methods to intuitively and efficiently perform fairly complex date math. If you're looking for a stress-free way to convert timestamps between different locations, I'd heartily recommend it to you. Take a look, and happy coding!