fix: fixed formula to ensure last 20 seconds of an epoch belong to current epoch

[nemo83]

fixes: #24

[matiwinnetou]

The only small thing I would ask if we can exactly document this as comment why we are adding two lines. Yes I know we have git blame and a person can look up the ticket but it maybe easier to have it in the code, what do you think?

[nemo83]

The only small thing I would ask if we can exactly document this as comment why we are adding two lines. Yes I know we have git blame and a person can look up the ticket but it maybe easier to have it in the code, what do you think?

Comments get stale, code is law, will split each line into two:

1. slot time
2. end of slot time
   This will make things more explicit