Hi,
why did you ignore the questions at the beginning?
But to be honest. I never saw a datepicker there, neither in version 3.x. nor in 5.x.
You get a datepicker when you edit a single participant, but not in the mass editing.
a. single
b. mass
As you see, there are different icons at the right.
So you have to enter the date in MySQL format "YYYY-MM-DD"
Anyway, you may file a bug report or a feature request.
Joffm