Dallas wrote:
c. Except as provided in paragraphs (d), (e), and (g) of this
section, no person may act as pilot in command of an aircraft unless,
since the beginning of the 24th calendar month before the month in
which that pilot acts as pilot in command, that person has --

Sorry... I can usually decode the FARs but my decoder ring broke
trying to unravel this one.

That sort of language is a result of someone trying to cram too many things
into one sentence. I've noticed a mix of good and bad writing styles in the
FAA regulations. But I doubt they'll rewrite that regulation just to make
it easier to understand.

Anyway, assume the pilot wants to act as PIC on December 24, 2009. One
month before December 2009 is November 2009. So 24 months before December
2009 is December 2007. So if the pilot had a BFR anytime on or after
December 1, 2007 (e.g. December 12, 2007) they may act as PIC any day of
calendar month December 2009. Hence till December 31, 2009.

I've been assuming that the BFR had to be accomplished like the FAA
usual "last day of the month". i.e. Last BFR December 12, 2007 -
Next BFR before December 31, 2009?

Your understanding appears correct and as far as I can tell, and is
equivalent to the tortured FAA language.