Days left until

Im trying to do a script that tell how many days is left until a specific date.

Ive come this far:

today = new Date(today.getYear() + 1900, today.getMonth(), today.getDate());
today = today.getTime();

thedate = new Date(2004, 7, 7);
thedate = thedate.getTime();

then something like:
daysleft.text = thedate - today;

but that returns: “NaN”