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)
And why don't you just use [``tz``](http://momentjs.com/timezone/docs/#/using-timezones/converting-to-zone/)? You'll also find an example in the [``timezone-service.js``](https://github.com/opf/openproject/blob/bf6d8ffc0061ba027de4c6b04dc1a83625e2ae89/app/assets/javascripts/angular/services/timezone-service.js#L34-L41).