Are using a microflow to create the object thats holds your date attribute? If so you can set the date time whichtever date you like in the create object action, if you then pass this along to an edit page with a date picker it should be prefilled.
You’ll have to change the attribute that you’ve linked to your date picker, once you assign your date to it and get the object to refresh in client you should see the UI change.
I suggest going through the learning courses in the academy section to cover more of the basics.
Hope this helps