Difference between dates

Question

I want to calculate the difference between two times, one of which is the current time, and the other is just in the format HH:MM, always in the future.

If I just subtra

Answer 1

Simply add a whole day to the difference if it is negative. Assuming that $now and $futuretime are timestamps (stored in seconds since midnight), simply do the following:

$diff = $futuretime - $now;
if ($diff < 0)
    $diff += 24 * 3600;

If they are in HH:MM format:

list($future_hours, $future_mins) = explode(":", $futuretime);
$futuretime = $future_hours * 60 + $future_mins;
list($now_hours, $now_mins) = explode(":", $now);
$now = $now_hours * 60 + $now_mins;
$diff = $futuretime - $now;
if ($diff < 0)
    $diff += 24 * 60;

Of course the latter case returns the difference in minutes, while the former returns the difference in seconds.

Edit: as Andy noted below in the comments, this is not a good solution if you care about changes in DST. See his solution for a better approach.