Doing such calculation isn’t possible within Ubidots, at least not natively. My suggestion would be to do as follow:
Create a Manual input widget with the the Date fields enabled. Learn more here.
Have the user enter the Date
From an HTML canvas widget, build your own logic so that it receives the date (in timestamp format) entered by the user, and compute the number of days since that date.
You might want to look a he below article about the HTML canvas widget