To calculate number of days excluding sundays

In Excel 2010 and later you can use NETWORKDAYS.INTL to count days between two dates excluding Sunday only, i.e.

=NETWORKDAYS.INTL(A2,B2,11)

In earlier versions of excel you can use this formula to get the same result

=B2-A2-INT((1-WEEKDAY(B2)+B2-A2)/7)

I would guess by taking the difference of days and dividing by 7 you would get the sunday count... probably round down

Something like this...
=RoundDown((Date1-Date2)-((Date1-Date2)/7),0)

All theory, but looking at that on here it logically makes sense...would have to verify though

An alternative:

Formula:

=SUMPRODUCT(--(WEEKDAY(ROW(INDIRECT(a1&":"&a2)))=1))

assuming your two dates are in A1 (first date) and A2 (second date). The 1 at the end specifies Sunday. You can exclude other days by changing it to 2 = Monday, 3 =Tuesday, etc.