Description:
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I was testing unixtime from php and delphi and there is a difference from one to another. 

Php makes unixtime from 12/31/1969 21:00:00. Isn't it a bug? Unixtime isn't from 01/01/1970 00:00:00 as you say in manual from function strtotime:

'...This means that e.g. dates prior to Jan 1, 1970 will not work on Windows, some Linux distributions, and a few other operating systems....'

If it's because the time, isn't it needed another argument to define it? 

For example:

If i make a script using unixtime using -0300 timezone and, for some reason, i move it to another system that uses timezone as GMT, some results will be different.

Reproduce code:
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<?                    
$unixtime = 0;
echo date("d/m/Y H:i:s", $unixtime) . "\r\n";
?>


Expected result:
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01/01/1970 00:00:00

Actual result:
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31/12/1969 21:00:00