I edit legal texts written in English by non-native speakers (in Belgium). The term 'within' is used constantly when speaking of a period between a given date and the deadline for doing something , as in "this law will enter into force within 30 days of its publication in the official gazette" or "the penalty must be paid within 15 days of receipt of the notice". I have noticed that people (even in directives by the European Union in the official English version) often use "within 30 days from the date of publication" and so on.

There was a discussion on the English usage forum asking whether "within 30 days after x) was ok and most people (to my mind rightly) said no, it should be of. The discussion there had to do with whether "within x days" always means AFTER or if it could also mean BEFORE the date in question - the example was a statement that the client has to notify the service provider within 10 days of moving to the new address" and whether that could be understood as 10 days before the move. I have never thought of this issue, assuming "within x period of time" was always after the date x.

Of course an alternative is to say "must be paid at the latest 15 days after receipt of the notice" but I would still like to know if I should be correcting "within x days from" to "within x days of".

In Merriam-Webster all examples of use of within in this sense has "of".

Any thoughts?