- round: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$date = strtotime('2014-01-15');
$daysbefor1979 = '25568';
$start = strtotime("1900-01-01");
echo $start;
$diff = ($date - $start);
$days = ($diff / 86400);
echo date('Y-m-d', jdtounix(gregoriantojd(12, 31, 1899) + 41603)).PHP_EOL;
echo round(($daysbefor1979+$days), 0);