zone_offset returns nil when it should return 0 for ISO8601 dates

I get the error

NoMethodError: undefined method `<' for nil:NilClass

In time.rb:156 when I am parsing ISO8601-format (eg. 2024-10-17T12:49:41.854Z) expiration dates of cookies in a Rails 7 application.

The error comes from the apply_offset method being passed a nil value in the off parameter instead of a number. That value is calculated by the zone_offset method. Something goes wrong in the comparison with ZoneOffset but I don't understand why.

I've made a quick and dirty refinement, defining the zone offsets in a variable inside the method and this one works as expected.

module TimeRefinements
  refine Time do
    class << Time
      def zone_offset(zone, year=self.now.year)
        z_offset = {
          'UTC' => 0,
          # ISO 8601
          'Z' => 0,
          # RFC 822
          'UT' => 0, 'GMT' => 0,
          'EST' => -5, 'EDT' => -4,
          'CST' => -6, 'CDT' => -5,
          'MST' => -7, 'MDT' => -6,
          'PST' => -8, 'PDT' => -7,
          # Following definition of military zones is original one.
          # See RFC 1123 and RFC 2822 for the error in RFC 822.
          'A' => +1, 'B' => +2, 'C' => +3, 'D' => +4,  'E' => +5,  'F' => +6,
          'G' => +7, 'H' => +8, 'I' => +9, 'K' => +10, 'L' => +11, 'M' => +12,
          'N' => -1, 'O' => -2, 'P' => -3, 'Q' => -4,  'R' => -5,  'S' => -6,
          'T' => -7, 'U' => -8, 'V' => -9, 'W' => -10, 'X' => -11, 'Y' => -12,
        }

        off = nil
        zone = zone.upcase
        if /\A([+-])(\d\d)(:?)(\d\d)(?:\3(\d\d))?\z/ =~ zone
          off = ($1 == '-' ? -1 : 1) * (($2.to_i * 60 + $4.to_i) * 60 + $5.to_i)
        elsif zone.match?(/\A[+-]\d\d\z/)
          off = zone.to_i * 3600
        elsif z_offset.include?(zone)
          off = z_offset[zone] * 3600
        elsif ((t = self.local(year, 1, 1)).zone.upcase == zone rescue false)
          off = t.utc_offset
        elsif ((t = self.local(year, 7, 1)).zone.upcase == zone rescue false)
          off = t.utc_offset
        end
        off
      end
    end
  end
end

Start at time.rb:156 and trace apply_offset with the ISO8601 value 2024-10-17T12:49:41.854Z, then inspect how zone_offset determines the offset for Z. Reproduce the cookie expiration parsing failure and confirm that zone_offset supplies a numeric offset so the NoMethodError no longer occurs.