class Rack::ConditionalGet

def to_rfc2822(since)

format), or nil if the string cannot be parsed.
Return a Time object for the given string (which should be in RFC2822 
def to_rfc2822(since)
  # shortest possible valid date is the obsolete: 1 Nov 97 09:55 A
  # anything shorter is invalid, this avoids exceptions for common cases
  # most common being the empty string
  if since && since.length >= 16
    # NOTE: there is no trivial way to write this in a non exception way
    #   _rfc2822 returns a hash but is not that usable
    Time.rfc2822(since) rescue nil
  end
end