When was Martial Law "Declared"?

Many history books state that martial law was declared by Ferdinand Marcos on 21 September 1972, the date that appears on Proclamation 1081. Those who lived through those times (not me!) will say, however, that the anniversary of martial law falls on September 22, if they were arrested on that day; or September 23, the day when Marcos appeared on TV to announce the imposition of martial law.

To complicate matters further, two months later, Marcos claimed to have signed Proclamation 1081 on September 17. So was it postdated or antedated? Or do we just rely on the date on the document? Fortunately, we have Manolo Quezon, who seems to have encoded Marcos's diary entries from January 1971 to September 1973 all by himself. [Aside: Why are these entries not available on the Marcos, NHI or RMAO websites?] Here's an excerpt from Marcos's entry for September 21:

Delayed by the hurried visit of Joe Aspiras and Nating Barbers who came from the Northern bloc of congressmen and senators who want to know if there is going to be Martial Law in 48 hours as predicted by Ninoy Aquino.

Of course Imelda and I denied it.

But Johnny Ponce Enrile, Gen. Paz, Gen. Nanadiego, Kits Tatad and I with Piciong Tagmani doing the typing finished all the papers (the proclamation and the orders) today at 8:00 pm.

This seems to imply that Proclamation 1081 was finalized on September 21, and must only have been signed afterward. But why would Marcos later claim that he signed it on September 17?

Anyway, assuming Proclamation 1081 was signed on September 21, should that be equated with "declaring" martial law? If perhaps all we had was the official document, and everyone who lived through those times were dead, then maybe we would have to settle for what was written on the document. But lots of books and articles are available attesting to the fact that people were arrested on the evening of September 22, and that Marcos announced the imposition of martial law on September 23. What exactly should be given more weight, the official document or other written and oral sources?

But the weirdest thing, though, is that I attended the Philippine Historical Association's National Conference last Saturday, 21 September 2007, which had as its theme "Controversies and Fallacies in Philippine History," and not once was any reference to the declaration of martial law made during the day-long event. The conference itself was quite enlightening about matters that occurred more than a century ago. But I wonder whether the absence of references to more recent events (aside from the extended discussion of textbook errors) was done intentionally.