When is "2 weeks from the vaccination dose" complete for entry to France?

Obviously, it's safer to use the later date. By inductive reasoning I would argue that the day of the vaccination should not be counted, but the same line of reasoning also leads to the conclusion that the relevant date is August 4th.

First, assume that "two weeks" is fourteen days (perhaps questionable for France, where they often use the expression quinze jours, "fifteen days," to mean two weeks, but I'll assume for now that the French-language rule is deux semaines).

Next, consider that if the requirement were "one day after the second dose" then the relevant date after a vaccination on 21 July would be 22 July. Therefore, "two days" would denote 23 July, "three days" 24 July, and so on, until you arrive at "ten days," denoting 31 July, "11 days," denoting 1 August, and eventually "14 days," which denotes 4 August.

After a quick search, I found a page on the French government site that uses "2 semaines," so there should be no need to worry about quinze jours.

However, as noted in a comment, it seems that at least Germany, for whatever reason, considers that one is fully vaccinated only as of the beginning of the fifteenth day after vaccination. I suppose that this is likely because during the earlier part of the fourteenth day after the vaccination, before the time at which you were vaccinated, you aren't considered fully vaccinated, and it is easier just to wait until midnight than to try to measure the 14 days down to the second.

Now that this wrinkle has been introduced, and without being able to find anything explicit about how France approaches this question, I would assume that they approach it as Germany does, and use August 5th.

The dose is legally effective on the 14th day after the injection. So if you had your second shot on a Wednesday, you can enter France on the Wednesday two weeks after.

Note that being vaccinated helps, but depending on where you're coming from and when you're travelling it might not be sufficient. As of today, the UK is considered an “orange” country, meaning that you also need a recent test, but you don't need pressing grounds for travel and you don't need to isolate after arrival. Beware that these rules change often and I do not intend to keep this answer updated.

The French government advice page defines “fully vaccinated” as “two weeks”, which unfortunately is imprecise.

• Two weeks after the second shot for two-shot vaccines (Pfizer, Moderna, AstraZeneca);
• Four weeks after the shot for one-shot vaccines (Johnson & Johnson);
• Two weeks after the shot for vaccines administered to people who have already had COVID-19 (only one dose is necessary).

Only vaccines authorized by the European Medicines Agency (EMA) will be accepted, i.e. Pfizer (Comirnaty), Moderna, AstraZeneca (Vaxzevria) and Johnson & Johnson (Janssen).

I can't find the applicable legal text, only the law for what is applicable once inside France, which is currently different. Once inside France, according to the décret 2021-699 du 1er juin 2021, as of today (the decree has already been revised several times),

Un justificatif du statut vaccinal est considéré comme attestant d'un schéma vaccinal complet (…)

a) S'agissant du vaccin “ COVID-19 Vaccine Janssen ”, 28 jours après l'administration d'une dose ;

b) S'agissant des autres vaccins, 7 jours après l'administration d'une deuxième dose, sauf en ce qui concerne les personnes ayant été infectées par la covid-19, pour lesquelles ce délai court après l'administration d'une dose ;

Translation: according to French law, you are considered fully vaccinated on the 7th day after your second injection of Pfizer, Moderna or AstraZeneca (or sole injection if you have had covid), and on the 28th day after your single injection of Janssen.

An earlier version of this decree stated “quatorze jours”, i.e. 14 days, and as far as I know at the time the rule was the same for border crossings and other purposes.

You may have seen references to “quinze jours”, which is a French colloquialism meaning two weeks. “Fifteen days” is not an accurate translation of “quinze jours” in a non-legal context. If you saw references to 15 days in English, that was a mistranslation.

A few considerations:

• Two weeks/14 days is necessarily somewhat imprecise. There are ways to be more specific like counting hours (as the rules do for tests) but the law was not phrased that way. This makes sense as the immune response is not expected to suddenly increase after exactly 336 hours.

• In keeping with this, enforcement is not very strict. Anecdotal reports (here and elsewhere) suggests it is sometimes completely absent (land borders inside the EU) or based on verbal declarations (Gagravarr's comment to the question). The point is also to push people to get vaccinated and tell them off the idea of traveling without a test the day after receiving their first shot.

• As the EU Digital COVID Certificate is rolled out, rules on entry will presumably increasingly be enforced using the “TousAntiCovid Verif” app, which could create an implicit definition. How the threshold is implemented is not documented and the source code is not available so I do not know how the app treats this situation. The most systematic enforcement I have witnessed or experienced is at the Gare du Nord in Paris but it was still mostly based on regular test certificates.

• If you wait for 14 full calendar days to have elapsed (say you get your second shot on Thursday July 1st and you travel on Friday July 16th), there is no doubt that you meet the requirement. There are several other reasonable interpretations that would allow travel from 12 AM or at some other point during the day of the 15th. In most cases, I expect it should be possible to convince a border guard to let you through but do you really need to risk a fine, additional stress or discussion with a carrier over this?

  If you are currently planning a trip, planning a journey on the 16th is therefore an easy, if potentially somewhat inconvenient, way to solve the problem. Incidentally, that's the definition of a “jour franc” in French contract law.

• Similarly, you always have the option of getting a test just before your trip (it doesn't need to be a PCR test).