A clock does not have values of

24 hours
60 min
60 sec
100 100th

for example a clock showing 59 seconds will change to 0 on the next second.

the calculation

if Seconds - Set_Sec <0 then
OT_Minutes = OT_Minutes - 1
OT_Seconds = 60 - Set_Sec + Seconds
endif

should be

if Seconds - Set_Sec <0 then
OT_Minutes = OT_Minutes - 1
OT_Seconds = 59 - Set_Sec + Seconds
endif

and the same for the other < comparisons.