Logical meaning of "within 30 days" compared to "in 30 or fewer days" or "fewer than 30 days"

As I understand it, the phrase "within 30 days" is the same as "in 30 or fewer days" not "fewer than 30 days." Is this correct?

(I've chosen fewer instead of less because days are a quantity that I can count individually, as opposed to having less time or energy for example.)

Solution 1:

Within 30 days = You have no more than 30 days. (By itself this does not clarify when the start date or end date is).

Within 30 days of the expiration date = You have a period of 30 days before the expiration date (inclusive).

You must file the form it during the 30-day period immediately before the expiration date = further clarifies that you must file the form inside of that 30 day just before the expiration date (inclusive).