The date selection by calandar function is not possible because the fields are not corresponding with the images.
Have a look at the video. It is recorded using the developer tools of firefox. Just follow the mouse movement.
This is present on every calendar function, it does not only appear in the date selection on creating a new event.

The placement is moved to the bottom and it is not possible to select any date correctly.
Does this depents on any "margin-top" value of your CSS?
Do you have seen this before or is this the first time?
How can I solve this