Last-Modified Header Validator

The Last-Modified header states when the resource last changed, and it underpins conditional revalidation: the client stores the date and, on the next visit, asks whether anything changed since. If nothing did, the response is short and bodiless. But the date format is rigid, and a date the client cannot parse makes all that saving disappear.

Paste the value and the page validates it against the required format: abbreviated English weekday, comma, two-digit day, abbreviated English month, four-digit year, time, and the literal GMT. None of that is negotiable — not the language, not the leading zeros, not the zone, which is always GMT even when the server sits elsewhere.

Three historical formats were once accepted, and the current specification says to generate only this one and accept the other two when reading, for compatibility. In practice, a server generating an old format meets a client that will not parse it, and revalidation quietly stops working — no error, just the resource being downloaded in full every time.