The Date Picker allows for text to be entered, it will highlight the field in RED:
but still allows the activity to be submitted, and no Date is set.
This is a Defect with the Date Picker. Engineering has corrected this and this fix will be included in an upcoming Public Fix pack.
The fix changes the functionality, you can no longer enter anything in the box, instead a small calendar will open requiring a date to be specified.
If you need this fix, please open a case with the support team and reference this Knowledge document, and Defect "DE591837 Date Picker allows for text to be entered, but ignores it."