If the current time is $t$, and the time $n$ hours into the future is $f$, using a $24$ hour clock, we have:
$t$ is the current time. $t+n$ is the future time. $f$ is also the future time. Thus:
$$t+n\equiv f\pmod{24}$$
using a twelve hour clock, this is instead simply $t+n\equiv f\pmod{12}$
If the current time is 2:00, and we want to know what the future time is 100 hours into the future, we have:
$f\equiv t+n\equiv 2+100\equiv 102\equiv 4\cdot 24+6\equiv 6\pmod{24}$
so the future time will read 06:00
If the current time is $t$ (currently unknown), and $100$ hours into the future the clock reads $2:00$ (the future time), we have:
$t+n\equiv f\pmod{24}$ implying
$t\equiv f-n\equiv 2 - 100\equiv -98\equiv -5\cdot 24 + 22\equiv 22\pmod{24}$
so the current time reads 22:00
Similarly, we could talk about time into the past.
If the current time is $t$ and the time $n$ hours before it into the past is $p$, using a $24$ hour clock we have:
$t$ is the current time. $t-n$ is the past time. $p$ is the past time. Thus:
$$t-n\equiv p\pmod{24}$$
If the current time is 12:00 and we are curious what time it was 45 hours into the past, we have:
$p \equiv t-n\equiv 12 - 45\equiv -33\equiv -2\cdot 24 + 15\equiv 15\pmod{24}$
thus, the time 45 hours in the past was 15:00
If the current time is unknown, $t$, but we know that 45 hours into the past the time had at that point read as 12:00, we have:
$t\equiv n+p\equiv 45+12\equiv 57\equiv 2\cdot 24 + 9\equiv 9\pmod{24}$
so, the current time is 9:00.
Again, in all of these examples, if you were using a twelve hour clock instead, you would use modulo twelve. In each case, you are asking "how much larger than the closest smaller multiple of twenty-four (or twelve as the case may be) is my number?"
If you are not keeping track of the date, you can simply use $(24+d+t)$ mod $24$ where $d$ is the time difference and $t$ is the time you are starting with. Your example would become $(24-4+00:00)$ mod $24$, which would be $(20+00:00)$ mod $24$, which would then become $20:00$.
If you are keeping track of the date, you will have to decrement it, as the extra 24 hours added to the time difference will push the date forward one day.
Best Answer
In non-modular arithmetic language:
After 12 hours, clock reaches back to 11. So we find number of "12" hours in 80 hours =$6\times12=72$
There's remaining $8$hours which pushes clock handle from 11 to 7 (11,12,1,2,....,7)