Does the word "of" in the context of an established point in time refer to before or after that established point in time?
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To use the phrase that you have used in the subject title, this would normally mean that payment is due within 3 months after the date of the meeting, i.e. within the 3-month period commencing on the date of the meeting.
In many cases, the sense would, in any event, be clear from the context:
The expression within three months establishes a period for payment with both a beginning- and an end-point. The issue is to determine if the meeting is intended to mark the beginning or the end.
The circumstance in which it is appropriate to specify the meeting date as the end point is if the payment is "for" the meeting (such as a booking fee). On the other hand, if the payment arises as a consequence of the meeting (such as a new levy that has been authorised), the payment period begins with the meeting date.
Since the payment period begins at the meeting option is far more common, that would be the usual interpretation.
I think that the sentence in the title is quite poor, but you can almost mathematically weigh up what it means. Within 3 months "of" a date can mean 3 months before or 3 months after, but from a practical point of view, it only matters about the latest date which sets a deadline for payment.
If the the word "from" was used instead of "of", this could only mean the future (in reference to the date), but again, in practical terms it only matters about the upper bound of the time span.