class HTTP::Retriable::DelayCalculator

def delay_from_retry_header(value)

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After
Spec for Retry-After header 
def delay_from_retry_header(value)
  value = value.to_s.strip
  case value
  when RFC2822_DATE_REGEX then DateTime.rfc2822(value).to_time - Time.now.utc
  when /^\d+$/            then value.to_i
  else 0
  end
end