You'll have to use this plugin to get the date:
on the start date save the UnixTimestamp (seconds since Jan 1, 1970) to webstorage for later, we'll call this start_time.
Then any following times you want to see how many days have passed you retrieve the saved time from webstorage and subtract the current UnixTimestamp. That will give you the number of seconds from one date to another. Divide by 60 to get minutes, 60 again to get hours, and finally also by 24 to get days.
days = (start_time - UnixTimestamp)/60/60/24