Hi Rita,
Could you please tell us which DatePicker you are using. Try to use DatePicker from OutsystemsUIWeb->Controls -> DatePicker.
For this DatePicker you have StartEmpty property available which will help to display No Date Selected by default.

NOTE: If you don't have this DatePicker available, try to take its reference from manage dependencies window from OutsystemsUIWeb->Controls -> DatePicker.
Hope this helps, Thanks :)