daf bit: Gittin 18
When a couple divorces, the woman must wait at least three months before
remarrying. This is so it is clear who the father is of any child born soon
after. Since a get (divorce document) can be written and delivered
later, perhaps with travel (if the husband is in a different city), from
when do we count the three months? Rav says we count from when the
get is delivered, and even if the bearer lingers on the road three
months before delivering it, she must still wait another three months.
R' Shmuel says we count from the time it is written, and we need not be
concerned because the husband has not been with her in the interval.
The halacha is that we count from when it is written. (18a)