Convert datetime coffeescript spec to ES6

## What does this MR do?
Converts the last remnant coffeescript file to ES6

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Because we aren't using coffeescript anymore 🎈 

## What are the relevant issue numbers?
#21472 

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6045