It starts from the date when you ceased to be a resident of the UK and became a resident of France.
Note that if you apply to your former UK council to be an overseas voter, or when you renew overseas voter registration (which is required annually), you are in any case asked what date you left the UK (the online form at gov.uk/register-to-vote asks for month and year).
An Electoral Commission spokesman said if in doubt you should check the date with the electoral services at the council. The 15 years run from the day when they removed you from the register of ordinary voters and added you to an overseas voter one.
He said that according to British election law you should not vote in any election taking place more than 15 years later. So, for example, even if you recently registered or re-registered as an overseas voter, you should theoretically not take part if in the meantime more than 15 years has elapsed.
Dr Susan Collard, senior lecturer in French politics at the University of Sussex who studies overseas voting, said: “The start date is nothing to do with your residence permit or tax. EROs [electoral registration officers] just care about what they have on record for when you left their [local] register. It relates to when you notified them you were leaving, though for some people that might have been quite a fluid thing.
“They might have been spending six months or more in France but still have felt based in the UK, but at some point they decided they had moved and if they were interested in keeping their vote, they would have told their ERO. Those that don’t think to do it drop off the register because they have not responded to the annual canvass.”
Dr Collard said that in many cases EROs write to people when the 15 years are up.
She said if you are no longer entitled, you should be taken off and should not receive a postal ballot or proxy vote, but if an election takes place just after you technically ceased to be eligible, it may depend on how strict council officers are about doing checks.
She added that she would suggest people vote if given the chance to do so.
However Dr Collard said it is possible that in some cases votes received from someone who is found to have been away more than 15 years are not counted.
You would not know for sure unless you go to the electoral office and ask to see the “marked register” (if the vote was by post it is the “postal register”), which has pencil marks on it showing whether each person voted.
Overseas voters are at the back, listed in alphabetical order.