Hi Romaniko,
I think you have a very good point, client side format date function doesn't have the same behavior as the server side function and you should report this.
As a workaround, I would not follow the previous suggestions here mentioned for making use of a server side action for several reasons:
- The advantage of using reactive applications is that you are not dependent from server side actions, you can use client ones which will help you on performance.
- Using server actions on onInitialize is a bad principle which can be seen on the comment automatically generate by OutSystems when you have onInitialize:

- Your format action won't work on your mobile app if you are offline since it cannot reach the server if you don't have internet.
Being that said, you can very easily "fix" this behavior with a client action.
I send you an OML with a client-side function you can use to achieve your goal.
Hope it helps.
Kind Regards,
João