Then you need to put this into the localstorage / webstorage and restore the date upon a new game session from there and compare.
It's pretty tricky to do for a beginner, I suppose, but this way, it will work.
Edit: Or maybe you want to store year, date, day, hours and minutes into different variables for easier handling. I believe that may be easier.
Edit: The plugin appearently also supports Date.Date2UnixTimestamp.