Timex.Format.FormatError - invalid format: { :formatter, :unsupported_token, :ms }

[simpers]

Steps to reproduce

Currently using Timex version 3.4.0 through github directly using { :timex, github: "bitwalker/timex", tag: "3.4.0" }.

I was writing a unit test for our time formatting when I stumbled upon this. In my actual case format_string is a constant in the app itself, and not defined directly in the unit test. But other than that it is the same code as I have. The directive %f works, but is not what we need.

• API function called
• Arguments provided

test "Test formatting a timestamp with %L", _state do
    format_string = "%FT%T.%LZ"
    time = "2018-01-01T00:00:00.000Z"
    |> Timex.parse!(format_string, :strftime)
    |> Timex.format!(format_string, :strftime)
    assert time == "2018-01-01T00:00:00.000Z"
end

Description of issue

• What are the expected results?
  As per the documentation describing the Strftime formatter there should be a directive, %L, that produces: milliseconds (000..999)

• If you are certain this is a bug in Timex, do you have references I
  can use to check the algorithm/logic against? For instance, a well-established
  date/time library in another language, a paper, test suite, etc. It is by no
  means a requirement, but is immensely helpful in fixing bugs, and provides me
  extra data to use when building out the test suite

I'm sure it is a bug as there is documentation on it (perhaps by mistake, but then the documentation has a bug, I suppose). It is also quite a common way of formatting a timestamp so I would expect it to exist, as it does in other libraries (not sure of which one, but on the Java side gson formats it this way currently in our environment, and I am trying to match it up to this).

[simpers]

Noticed that there was a pull request #433 regarding this issue already. Didn't find it initially. So I guess this is a duplicate.