Good point! If you're going to pattern match in the initial string then who even needs the second function?

    def convert("12:" <> <<m::bytes-size(2)>> <> ":" <> <<s::bytes-size(2)>> <> "AM") do
      "00:#{m}:#{s}"
    end

    def convert(<<h::bytes-size(2)>> <> ":" <> <<m::bytes-size(2)>> <> ":" <> <<s::bytes-size(2)>> <> "PM") do
      h = 12 + String.to_integer(h)
      "#{h}:#{m}:#{s}"
    end

    def convert(<<h::bytes-size(2)>> <> ":" <> <<m::bytes-size(2)>> <> ":" <> <<s::bytes-size(2)>> <> "AM") do
      "#{h}:#{m}:#{s}"
    end

Whether that's an improvement or not is another matter :)