PHP : Dates and Times - [3.3] Converting Time and Date Parts to an Epoch Timestamp

3.3.1 Problem

You want to know what epoch timestamp corresponds to a set of time and date parts.

3.3.2 Solution

Use mktime( ) if your time and date parts are in the local time zone:

// 7:45:03 PM on March 10, 1975, local time
$then = mktime(19,45,3,3,10,1975);

Use gmmktime( ) if your time and date parts are in GMT:

// 7:45:03 PM on March 10, 1975, in GMT
$then = gmmktime(19,45,3,3,10,1975);

Pass no arguments to get the current date and time in the local or UTC time zone:

$now = mktime();
$now_utc = gmmktime();

3.3.3 Discussion

The functions mktime( ) and gmmktime( ) each take a date and time's parts (hour, minute, second, month, day, year, DST flag) and return the appropriate Unix epoch timestamp. The components are treated as local time by mktime( ), while gmmktime( ) treats them as a date and time in UTC. For both functions, a seventh argument, the DST flag (1 if DST is being observed, 0 if not), is optional. These functions return sensible results only for times within the epoch. Most systems store epoch timestamps in a 32-bit signed integer, so "within the epoch" means between 8:45:51 P.M. December 13, 1901 UTC and 3:14:07 A.M. January 19, 2038 UTC.

In the following example, $stamp_now is the epoch timestamp when mktime( ) is called and $stamp_future is the epoch timestamp for 3:25 P.M. on June 4, 2012:

$stamp_now = mktime( );
$stamp_future = mktime(15,25,0,6,4,2012);

print $stamp_now;
print $stamp_future;
1028782421
1338837900

Both epoch timestamps can be fed back to strftime( ) to produce formatted time strings:

print strftime('%c',$stamp_now);
print strftime('%c',$stamp_future);
Thu Aug  8 00:53:41 2002
Mon Jun 4 15:25:00 2012

Because the previous calls to mktime( ) were made on a computer set to EDT (which is four hours behind GMT), using gmmktime( ) instead produces epoch timestamps that are 14400 seconds (four hours) smaller:

$stamp_now = gmmktime( );
$stamp_future = gmmktime(15,25,0,6,4,2012);

print $stamp_now;
print $stamp_future;
1028768021
1338823500

Feeding these gmmktime( )-generated epoch timestamps back to strftime( ) produces formatting time strings that are also four hours earlier:

print strftime('%c',$stamp_now);
print strftime('%c',$stamp_future);
Wed Aug  7 20:53:41 2002
Mon Jun 4 11:25:00 2012