As someone who writes Elixir full-time in a product with deeply complex date / time needs (spanning multiple timezones, sources of time, multiple meanings of 'month', etc), I can say with some conviction that the the simplicity of Elixir's DateTime module is most definitely a feature. The lack of something as transparent in Rails was / is a huge part of what makes our team so productive since porting the product to Elixir; NaiveDateTime alone has saved us from more bugs than I care to think about. Between Elixir core and Timex, they've definitely solved the harder corners of time as well as any other library or platform I've seen.
FWIW, timezone handling was added in v1.8 or v1.9. At this point I think Elixir has ported all features from Timex, except date parsing and date formatting.
However sometimes I feel I only use it because of the weird default (non-)handling of timezones by DateTime.