getTime() etc. not working in Acrobat XI - help?

Hello,

I have been using getTime() to work out the number of days between 2 dates.

//set the variables

var oneDay = 24*60*60*1000;

var firstDate = new Date(from_date.rawValue);

var secondDate = new Date(to_date.rawValue);

//only if the 2 date fields aren't null

if ((from_date.rawValue!=null)&&(to_date.rawValue!=null)){

num_days.rawValue = Math.abs((firstDate.getTime() - secondDate.getTime())/(oneDay));

}

It has been working well in lots of versions of Adobe Reader (& other PDF readers) but not in Acrobat Reader XI.

I have tested getYear() and getMonth() and they don't work either!.

I'm very confused and wonder whether anyone else has this problem? (Just in Acrobat Reader XI).

Thanks