$gmt1 = strtotime('1970-01-01');

 $gmt2 = date('Y-m-d H:i:s',$gmt1);

 $gmt3 = strtotime('1970-01-02');

 $gmt4 = date('Y-m-d H:i:s',$gmt3);

 $gmt5 = strtotime('1970-01-01',$gmt3);

 9

 echo '<br /><br />--gmt1----<br />';

 print_r($gmt1);

 echo '<br /><br />--gmt2----<br />';

 print_r($gmt2);

 echo '<br /><br />--gmt3----<br />';

 print_r($gmt3);

 echo '<br /><br />--gmt4----<br />';

 print_r($gmt4);

18 echo '<br /><br />--gmt5----<br />';

19 print_r($gmt5);