When does the validity period start for a domain renewed early?

If you renew a domain before its expiration date (for example a few months in advance, or for several years at once), the extra period is added to the current expiration date, not to the payment date.

For example, if a domain expires on December 31 and you renew it for another year in October of that same year, the new expiration date will be December 31 of the following year — not October of the following year.

This means you never lose time by paying in advance: an early renewal simply moves the payment moment earlier, without shortening the validity period you've already paid for.