I have been using a home grown logbook package for 15 years now and the problems of Hours and Minutes (base 60) math is a sod. Particularly as the numbers grow. (Rounding errors etc). The only successful tactic I found (and it is totally reliable on even the most pedestrian system and software) - is to go back to minutes and do the Maths at that point and then return the answer to HH.MM format.
Here are Excell type routines : Assume HH.MM in A1
B1 : =(INT(A1)*60)+((A1-INT(A1))*100) ' This gets you MM
To recombine B1 to HH.MM
C1 : =(INT(B1/60))+(MOD(B1,60)/100) ' This gets you HH.MM
Note HH.MM format with a decimal point - NOT a colon.
Just put the Minutes into a seperate column and do all the maths there. Enjoy...
D&G