Parse a Retry-After header into a delay in milliseconds: the delta-seconds form
(Retry-After: 120) or the HTTP-date form (Retry-After: Wed, 21 Oct 2026 07:28:00 GMT;
a date in the past is 0). Undefined when the header is absent or unparseable, so the
caller falls back to its own backoff. Only an IMF-fixdate reaches Date.parse: V8 reads
"1.5" or "-5" as dates in 2001.
Parse a
Retry-Afterheader into a delay in milliseconds: the delta-seconds form (Retry-After: 120) or the HTTP-date form (Retry-After: Wed, 21 Oct 2026 07:28:00 GMT; a date in the past is 0). Undefined when the header is absent or unparseable, so the caller falls back to its own backoff. Only an IMF-fixdate reachesDate.parse: V8 reads "1.5" or "-5" as dates in 2001.