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

Apparently with just bare times PHP is assigning the date December 31, 1969. When I ran this:

echo date('F j, Y H:i:s', $t1) . "\n";
echo date('F j, Y H:i:s', $t2) . "\n";
echo date('F j, Y H:i:s', $t3) . "\n";
echo date('F j, Y H:i:s', $t4) . "\n";

I got this:

December 31, 1969 03:15:00
December 31, 1969 01:00:00
December 31, 1969 02:00:00
December 31, 1969 09:00:00

Remember that strtotime returns a UNIX timestamp, which is defined as the number of seconds since January 1, 1970. By definition a UNIX timestamp refers to a specific month/day/year, so despite the name strtotime is not really intended for bare times without dates.