Meets #14520: Timeline displays work packages with a diff of one day

[``* `#14520` Timeline displays work packages with a diff of one day``](https://www.openproject.org/work_packages/14520)
I guess this can become ``moment(this.start_date).toDate()`` (see [the doc](http://momentjs.com/docs/#/displaying/as-javascript-date/) for details). Please consider all the other places too.