touble with a time calculation

[richard]

its never going to work if you don't allow for the negative carry (borrow) , subtracting time is no different to subtracting base (16 12 10 8 2 ) numbers except that hours are base 24 , minutes and seconds are base 60 . have a look again at my example note the order of calculation also

[richard]

i'll say it again if the el1 time data vars update in real time via an isr (as I suspect they may) the whole process is flawed unless you can fix (stop the clock) the el1 time vars for the duration of the subtraction .