Time stamps

I have this table where i get some time stamp called start and end, i split those up into more tabels with start_hour,start_min,end_hour,end_min. There is a time between 12.00 and 12.30 where the people goes to break if they work over the break they get 30 min less in thier total worktime, if they start doing break or end during break i want to make a transformation that subtracts the time spent if they start thier shift in the break. I have 2 fields called Start_in_lunch and End_in_work. I made this into a boolean conditions so if they start or end in lunch it should trigger, how can i do this. I tried some DateDiff with some Timefromparts but with no luck. The simple solution saying Start-30 and end-30 works but doing that devided by 60 does not work. Any good ideers