calculation the number of days using two date fields

You can't using simplified field notation. Fortunately you can use JavaScript:

// Custom Calculate script

// Get the input field values
var s1 = getField("Date1").valueAsString;
var s2 = getField("Date2").valueAsString;

if (s1 && s2) {// Convert date strings to date objects
var d1 = util.scand("mm/dd/yyyy", s1);
var d2 = util.scand("mm/dd/yyyy", s2);

// Set this field value to the difference in days
// 864e5 = 86400000 = 1000 * 60 * 60 * 24 = number of milliseconds in a day
event.value = Math.abs(Math.round((d2 - d1)/864e5));

} else {
// Blank this field if either inputs are blank
event.value = "";
}


You may need to use a different format string in the util.scand lines so they match the format of your fields.

You can't. It requires a more complex script.
Basically you need to convert both values to a Date object, and then you can calculate the difference between them.

Thank you for your response.
Can I send you the document?
I dont know how to input this in PDF for java script.
my email is loulikesthis [at] gmail [dot] com.
If you send your email I'll send you the document.
This will show me how to input Java script in a document.

There are a number of tutorials here that cover this in greater detail. The following two will get you started:

http://acrobatusers.com/tutorials/2006/form_calculations

http://acrobatusers.com/tutorials/2006/date_time_part1