PHP, use strtotime to subtract minutes from a date-time variable?

I need to subtract 45 minutes from the date-time variable in PHP.

The code:

$thestime = '2012-07-27 20:40';
$datetime_from = date("Y-m-d h:i",strtotime("-45 minutes",strtotime($thestime)));
echo $datetime_from;

returns the result 2012-07-27 07:55.

It should be 2012-07-27 19:55, though. How do I fix this?