Conversion of Time

[#1]

I want to calculate total working hours of a faculty

Faculty session start time is given and session End  time is given

i have calculated session duration by subtracting start time from end time.

now i want to calculate total hours and minutes a faculty has taken the lecture in 2013 year but time has limitation of 24 hrs(24:00:00) so its useless here

for e.g faculty faculty id : 20 has taken lecture of 90 Hrs and 30 min in year 2013 ( assumed value)

what expression i should use for calculating Session duration ??

[#2]

Thank you all

I am very grateful to all you guys..