Here’s what Garner’s Modern American Usage has to say about anniversary:
anniversary (= the day of the year on which an event occurred in a previous year) is today used informally to denote a milestone in months or even weeks. That usage has become increasingly common, perhaps because there is no convenient equivalent for terms shorter than a year (milestone is close, but it doesn’t connote observance and recurrence the way anniversary does). Considering the word’s tight association with “year,” however, the loose usage is subject to criticism and should be avoided if possible.
Drafters are prone to this loose use of anniversary—”month anniversary” occurs in 2,818 contracts filed on the SEC’s EDGAR system as material contracts during the past year. (That’s a lot.) Eliminating this usage results in prose that is not only more logical but also more concise. Here’s an example:
The Company shall redeem this debenture in 24 equal installments of principal and accrued interest monthly beginning on the one (1) month anniversary following [read one month after] the First Closing Date.
But in terms of boneheadedness, I don’t think you can beat 12-month anniversary, as in ending on the 12-month anniversary of that termination date. The adjectival phrase 12-month is redundant!
A month anniversary? A remarkable misuse of the English language…
“Month”, of course, being defined to mean a calendar month, and not lunar, or other.
(Excellent site)
Is there a single word with the same relationship to “month” that the word anniversiary has to “year”?
Todd: I’m afraid there is no such word. But doing without poses no real challenge. Ken
I always try to avoid the expression “x month anniversary”.
It’s not only a solecism but in some cases can lead to confusion and, because the number of days in a month varies, can result in a change in the calculation of a payment obligation simply on the basis of which particular calendar months are included. The difference between a 28 day month and a 31 day month can have significant economic consequences and can lead to an arbitrary variation of several days. If you’re talking about measuring sales, for example, during a specified period, such a difference of just a couple days can significantly affect in the sales figure calculation. That’s especially true if the end of the normal sales cycle happens to fall just a day or two before or after the specified monthly “anniversary” date.
What’s so complicated about simply specifying that a specified calculation or measuring period shall be computed over a 60 or 90 or other specified period of days following the closing or other triggering event?
At least that way you have clarity.