Created Dates.php for date difference calculations.

dateDiff function example usage
strtotime examples:
echo dateDiff("2010-01-26", "2004-01-26") . "\n";
echo dateDiff("2006-04-12 12:30:00", "1987-04-12 12:30:01") . "\n";
echo dateDiff("now", "now +2 months") . "\n";
echo dateDiff("now", "now -6 year -2 months -10 days") . "\n";
echo dateDiff("2009-01-26", "2004-01-26 15:38:11") . "\n";
Output:
6 years
18 years, 11 months, 30 days, 23 hours, 59 minutes, 59 seconds
2 months
6 years, 2 months, 10 days
4 years, 11 months, 30 days, 8 hours, 21 minutes, 49 seconds
UNIX timestamp and precision examples
echo dateDiff(time(), time()-1000000, 1) . "\n";
echo dateDiff(time(), time()-1000000, 3) . "\n";
echo dateDiff(time(), time()-1000000, 6) . "\n";
Output:
11 days
11 days, 13 hours, 46 minutes
11 days, 13 hours, 46 minutes, 40 seconds
Converting text format back to UNIX timestamp example
$time1 = time();
$time2 = $time1-10000000;
echo $diff = dateDiff($time1, $time2) . "\n";
echo $time1 . "\n";
echo strtotime(" +".$diff, $time2) . "\n";
Output:
3 months, 23 days, 17 hours, 46 minutes, 40 seconds
1264514564
1264514564