Wait, was that really on March 4?

So today I go to the La Poste site with the intention of ordering the recent Tristan Corbière stamp that I mentioned recently. I finally find the stamp shop at La Poste (from the first link above, click on "Visiting France" and then "Stamp Shop") and go down to the stamp that I want, but there it says the stamp was issued on March 7. Linn's said it was on March 4, so which is correct? I've sent a couple inquiries on this, but went ahead and ordered the stamp. Even if it was issued on March 7, I can put it in my worldwide collection anyways.

Update (Oct 19, 2011) - I sent an email to Linn's asking about this and I got the following response from Marty Frankevicz, the New Issues Update editor:

"Both [dates] are technically correct, but when it can be determined, Scott uses the more helpful date, the date shown on the first day cancel. This information usually is shown in La Poste's Phila@poste catalogue, which we get through the mail. It is not shown on their website, which states the first dates of general sale, which for France, is generally a few days later than than the FDC date."

So the stamp was issued on March 4 and its first day of sale was March 7.  This introduces another aspect of anniversary date collecting that I hadn't thought of before.  But after attending NTSS 2011 last June, where there was a first day of sale ceremony for the Mark Twain stamp, I should have thought of this.  I'll have to add this info to The List somehow.  I wonder if I can get a list of first day of sale dates to find even more stamps that should be in my March 4 collection...