Excel 2007 : Need help on an IF formula IF function problem

I am fairly new to this and have created an IF function to find the days between two dates. The date fields may not always be populated because they represent dates of completion in a continuum. If the date fields are not populated I want an "NA" to appear, if a date appears in only the first field I want to the days to be the days to today's date from the date in the field, and if both fields are populated I want the days between the two dates.

=IF(OR(E:E>0&F:F>0,E:E >0&F:F=0), IF(F:F=0, DAYS360(E:E,TODAY(),FALSE), DAYS360(E:E,F:F, FALSE)), "NA")

The days appear in both cases but the "NA" does not appear if the fields are blank instead the number 40359 appears.

Please help.