Don't you need SUM(INT.... rather than INT(SUM.... ? I think it will make a difference with more than 2 times to add up.
You need to sum the INTs then sum the decimalised fractions (value - INT(value)). Then add the INT of the summed fractions to the INT total, then undecimalise the fraction and add it to the INT total - I think.
I may know someone who's done hrs and minutes in excel?