Attendance - Counting minutes and hours

Hello,
there's one thing I've never got to work and which stopped me from using iDoceo. How can I count the delayed minutes and hours (if a student isn't absent the whole day) in the attendance sheet?

I want a column that gives me the sum of hours an minutes (see the attached image).

Thanks!
Christian

PS: Don't know why I can't attach a jpg-file?! --> Got it, there's a bug when using Safari.