I would think you would have to separate out the hours and minutes and convert them to numbers.  The solution below should work, but there may be a better solution.

numbervar hours:=tonumber(left({timetext},instr({timetext},":")-1));
hours*60+tonumber(right({timetext},2));

Of course the {timetext} field would replaced by your field.  This formula just returns the total number of minutes.  You can add code to do the division by three if that is all you want to do with the number.

I hope this helps.