Whether
they’re being used to display the current server time or record the exact
instance of a database entry, PHP’s date and time functions are a critical part of
any Web developer’s toolkit. Any tool, however, is only as good as the
craftsman wielding it; and that’s where this document comes in. The following
table (Table A) outlines the most
important date and time functions available in PHP, providing clear
explanations of where each should (and should not) be used. So keep reading—you
just might learn something new!

Table A

Function

Explanation

Example
checkdate($month,
 $date, $year)

This
function returns true if the values supplied to it make up a valid date. For
example, this function will return false on illegal dates like 31-Feb-05.

Use this
function to validate dates and check that they exist before using them in a
calculation or saving them to a database.

 <?php
// returns false
echo checkdate(2,30,2005) ? “valid” :
“invalid”;

// returns true
echo checkdate(4,6,2010) ? “valid” :
“invalid”;
?>
getdate($ts)

When
used without an argument, this function returns the current date and time as
an associative array. Each element of the array represents a particular
component of the date/time value. An optional timestamp argument can be
passed to the function to obtain date/time values corresponding to that
timestamp.

Use this
function to obtain a series of discrete, easily-separated date/time values.

 <?php
// get date as associative array
$arr = getdate();

echo “Date is ” . $arr[‘mday’] . ” ” .
$arr[‘weekday’] . ” ” . $arr[‘year’];

echo “Time is ” . $arr[‘hours’] . “:”
. $arr[‘minutes’];
?>
mktime($hour,
$minute,
$second,
$month, $day,
$year)

This
function does the opposite of the getdate() function:
it generates a UNIX timestamp (the number of seconds elapsed since Jan 1 1970
00:00 GMT) from a series of date and time values. When used without any
arguments, it generates a UNIX timestamp for the current time instant.

Use this
function to obtain a UNIX timestamp for a time instant. This timestamp format
is commonly used in many databases and programming languages.

 <?php
// returns timestamp for 13:15:23
7-Jun-2006
echo mktime(13,15,23,6,7,2006);
?>
date($format,
$ts)

This
function formats a UNIX timestamp into a human-readable date string. It is
one of the most versatile functions in the PHP date/time API, and can be used
with a wide variety of modifiers to convert integer timestamps into exactly
the string format you require.

Use this
function when formatting dates and times for display.

 <?php
// format current date
// returns “13-Sep-2005 01:16 PM”
echo date(“d-M-Y h:i A”, mktime());
?>
strtotime($str)

This
function converts a human-readable English date/time string into a UNIX
timestamp.

Use this
function to convert non-standard date/time strings into standard, easily
portable UNIX timestamps.

 <?php
// returns 13-Sep-05
echo date(“d-M-y”,
strtotime(“today”));

// returns 14-Sep-05
echo date(“d-M-y”,
strtotime(“tomorrow”));

// returns 16-Sep-05
echo date(“d-M-y”, strtotime(“today +3
days”));
?>
strftime($format,
 $ts)

This
function formats a UNIX timestamp into a date string suitable for the current
locale, as previously defined with setlocale().

Use this
function to create date strings compliant with the current locale settings.

 <?php
// set locale to France (on Windows)
setlocale(LC_TIME, “fra_fra”);

// format month/day names
// as per locale setting
// returns “septembre” and “mardi”
echo strftime(“Month: %B “);
echo strftime(“Day: %A “);
?>
microtime()

This
function returns the number of seconds and microseconds elapsed between the
current time and Jan 1 1970 00:00 GMT.

Use this
function when benchmarking a particular code block, to more precisely measure
how long it takes to execute.

 <?php
// get starting value
$start = microtime();

// run some code
for ($x=0; $x<1000; $x++) {
    $null = $x * $x;
}

// get ending value
$end = microtime();

// calculate time taken for code
execution
echo “Elapsed time: ” . ($end –
$start) .” sec”;
?>
gmmktime($hour,
$minute,
$second,
$month, $day,
$year)

This
function generates a UNIX timestamp from a series of date and time values
expressed in GMT. When used without any arguments, it generates a UNIX
timestamp for the current GMT time instant.

Use this
function to obtain a UNIX timestamp for a GMT time instant.

 <?php
// returns timestamp for 12:25:23
9-Jul-2006
echo gmmktime(12,25,23,7,9,2006);
?>
gmdate($format,
$ts)

This
function formats a UNIX timestamp into a human-readable date string. This
date string is expressed in GMT (not local time).

Use this
function when representing a timestamp in GMT.

 <?php
// format current date into GMT
// returns “13-Sep-2005 08:32 AM”
echo gmdate(“d-M-Y h:i A”, mktime());
?>
date_default_
timezone_set
($tz)  and  date_default_
timezone_get()

This
function sets and retrieves the default time zone for all subsequent
date/time function calls.

Note: This
function is only available in PHP 5.1+

Use this
function as a convenient shortcut to set a time zone for subsequent date
operations.

 <?php
// set timezone to UTC
date_default_timezone_set(‘UTC’);
?>