Простая арифметика не устраивает?

Sumo 07.11.2013 09:57 / 07.11.2013 10:03

$total(15)
$start[^date::create[2013-11-07 13:00:00]]
$stop[^date::create[2013-11-07 13:15:00]]
^if(^eval($stop - $start)[%.4f] >= ^eval($total/(24*60))[%.4f]){...}