Change Date Format

Change Date Format

I am trying to change the format of a date field to mm/dd/yyyy. I created a function to do that

string dateformat.ChangeDateFormat(date inputdate)
{
mon = input.inputdate.getMonth();
dat = input.inputdate.getDay();
year = input.inputdate.getYear();
outputstr = mon + "/" + dat + "/" + year;
return outputstr;
}

and this script:

thisapp.dateformat.ChangeDateFormat(input.Purchase_Date );

But it doesnt work

Can you help?