[BUGFIX] fixes UI bug in safari and firefox

__Tasks:__

- [x] fix serializatioin of datetime
- [x] gather feedback

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | -
| Fixed tickets | -
| BC Breaks     | -
| Doc           | -

__Requires:__
- https://github.com/sulu-cmf/SuluSalesCoreBundle/pull/9