Dynamic Selection in Listbox to create dynamic Bookmark

[ManuelRühl]

I need a dynamic selection in a field to create a dynamic bookmark:

I tried to enter in the listbox:

=if(date(floor(monthend(today(),-1)))=DateField,1,0)

Works perfectly, 31.08.2017 is the result. But i can't save it into a bookmark, because when I press enter, 31.08.2017 is saved in the selection. So it's not dynamic in the Bookmark.

Flagging the DateField is possible, but selecting directly on the field would be better for me.