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

Because strtotime() outputs the number of seconds relative to the second argument (in your case, the Unix epoch (December 31, 1969 19:00:00)).

The negative numbers is expected because "3:15:00" is 56700 seconds before the Unix epoch.

Answer 2

If you want to do something like that, I think you want to just do some math on the time strings themselves and convert them to a number of seconds, like this:

<?php
function hmstotime($hms)
{
    list($hours, $minutes, $seconds) = explode(":",$hms);
    return $hours * 60 * 60 + $minutes * 60 + $seconds;
}
?>

Answer 3

Try it without the second parameter. That's supposed to be a timestamp for the returned time to be relative to. Giving it 0 means you're asking for a timestamp relative to the Unix epoch.

In response to your comment:

It's not documented functionality, but I use strtotime("HH:MM") all the time, and it returns a timestamp relative to the current time. I guess if you want to be sure though, you could do this:

strtotime("3:15:00",time());

Answer 4

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.

Answer 5

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...