Since not working?

``` 
git2json --since 2018-01-01
usage: git2json [-h] [--git-dir GIT_DIR]
git2json: error: unrecognized arguments: --since 2018-01-01
```
Also other date formats and "this year" (which git log supports) don't work. -h also doesn't mention since. Installed today via pip, maybe that version is outdated?