Fix calculation of booked time
Fix calculation of booked time

... which would end up being negative for booking starting before
working hours.

mvidalgarcia/indico,OmeGak/indico,indico/indico,mvidalgarcia/indico,mic4ael/indico,pferreir/indico,mic4ael/indico,mic4ael/indico,indico/indico,indico/indico,mvidalgarcia/indico,indico/indico,OmeGak/indico,pferreir/indico,ThiefMaster/indico,ThiefMaster/indico,DirkHoffmann/indico,mvidalgarcia/indico,OmeGak/indico,mic4ael/indico,DirkHoffmann/indico,ThiefMaster/indico,DirkHoffmann/indico,ThiefMaster/indico,DirkHoffmann/indico,OmeGak/indico,pferreir/indico,pferreir/indico