Suppose that I have datetime like 2013-01-16-0900. How can I subtract a length of time - say, 30 minutes?
This will subtract 30 minutes from the supplied timestamp.
echo date( 'Y-m-d-Hi', strtotime( $timestamp ) - 30 * 60 );
$timestamp = '2013-01-16-0900' in this example.
Is it stored as a string exactly like you posted, or is it actually a unix timestamp? If it's a string you can run strtotime on it then subtract 1800 (seconds) from it, and then convert it back with date(). That would probably be the easiest way.
I have a strong preference towards using PHP's built in DateTime Module and adding or subtracting time using PHP's built in DateInterval Module. This allows you to perform reasonably complex date/time math and still respect the rules of time.
Also of note is DateTimeZone
Example Time
$dt = new DateTime("2013-01-16-0900", new DateTimeZone("America/Los_Angeles"));
$dt->setTimeZone("America/New_York");
$dt->sub(new DateInterval("PT30M"));
print $dt->format('Y-m-d H:i:s');
There is a bit of a learning curve to assembling time interval strings but once you figure it out, you should be able to write a re-usable function.