php mktime for epoch returns -3600, not 0

Question

If I run the following in PHP:

echo mktime(0,0,0,1,1,1970);

the returned value is -3600, not 0 as I expected.

The server is UK based, it's currently 21 Sep (i.e. BST summertime) (though I wouldn't expect this to affect the epoch timestamp) and per php.info: "Default timezone Europe/London".

Setting the daylight saving time flag also, as follows, gives:

echo mktime(0,0,0,1,1,1970,0); (i.e. the correct DST flag, 0 as 1 Jan not DST/BST) returns -3600

echo mktime(0,0,0,1,1,1970,1); (the incorrect flag - setting 1 Jan as DST) returns -7200

echo mktime(0,0,0,1,1,1970,-1); (i.e. DST flag not set - left to PHP to decide) returns -3600

Does anyone know why the epoch would be returned as -3600, not 0, please?

Answer 1

When it was midnight on Jan 1st 1970 in British Summer Time, it was one hour to midnight in Greenwich Mean Time. Try setting the time zone to UTC instead:

date_default_timezone_set('UTC'); // or just change php.ini

Answer 2

mktime() is based on your current timezone. If you want to create a timestamp based on GMT you have to use the gmmktime() function.

gmmktime(0,0,0,1,1,1970)

code on ideone

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Resources :

• php.net - gmmktime()