Strange behavior of PHP time math: Why is strtotime() returning negative numbers?

Question

I\'m trying to do some very basic time math - basically, given inputs of time and distance, calculate the speed. I chose to use strtotime() to convert the time inputs into s

Answer 1

strtotime() without a second argument gets the time from the supplied string and fills in the blanks from the current date:

echo date('Y-m-d H:i:s', strtotime("3:15:00"));
-> 2009-06-30 03:15:00

With a second argument it calculates the date relative to the second argument:

echo date('Y-m-d H:i:s', strtotime("3:15:00", 0));
-> 1970-01-01 03:15:00

To calculate the difference between two timestamps in seconds, you can just do this:

echo strtotime("3:15:00") - strtotime("3:00:00");
-> 900

Edit: Of course taking into account which is the bigger number:

$t1 = strtotime("3:15:00");
$t2 = strtotime("3:30:00");

$diff = max($t1, $t2) - min($t1, $t2);
$diff = abs($t1 - $t2);

Or something of that nature...