Check this post : http://success.salesforce.com/questionDetail?qid=a1X30000000KC20EAG (look all way below for the comment posted by NZgon)
I tried the code and works too just in case :
This gives you the difference between 2 days as number excluding the weekends ( make sure the formula field is a number field)
CASE(MOD( StartDate__c - DATE(1985,6,24),7),
0 , CASE( MOD( EndDate__c - StartDate__c ,7),1,2,2,3,3,4,4,5,5,5,6,5,1),
1 , CASE( MOD( EndDate__c - StartDate__c ,7),1,2,2,3,3,4,4,4,5,4,6,5,1),
2 , CASE( MOD( EndDate__c - StartDate__c ,7),1,2,2,3,3,3,4,3,5,4,6,5,1),
3 , CASE( MOD( EndDate__c - StartDate__c ,7),1,2,2,2,3,2,4,3,5,4,6,5,1),
4 , CASE( MOD( EndDate__c - StartDate__c ,7),1,1,2,1,3,2,4,3,5,4,6,5,1),
5 , CASE( MOD( EndDate__c - StartDate__c ,7),1,0,2,1,3,2,4,3,5,4,6,5,0),
6 , CASE( MOD( EndDate__c - StartDate__c ,7),1,1,2,2,3,3,4,4,5,5,6,5,0),
999)
+
(FLOOR(( EndDate__c - StartDate__c )/7)*5)
EDIT TO ANSWER IF THE FIELDS ARE DATE TIME VALUES: As per the posting above change all the date values to DateVALUE(Date) in short change all the startDate_c to DATEVALUE(startdate_c) and EndDate_c to DATEVALUE(EndDate_c), doing replace all in notepad++ will avoid the hazzle of figuring which bracket you missed.
CASE(MOD( DATEVALUE(startDate__c) - DATE(1985,6,24),7),
0 , CASE( MOD( DATEVALUE(EndDate__c) - DATEVALUE(startDate__c) ,7),1,2,2,3,3,4,4,5,5,5,6,5,1),
1 , CASE( MOD( DATEVALUE(EndDate__c) - DATEVALUE(startDate__c) ,7),1,2,2,3,3,4,4,4,5,4,6,5,1),
2 , CASE( MOD( DATEVALUE(EndDate__c) - DATEVALUE(startDate__c) ,7),1,2,2,3,3,3,4,3,5,4,6,5,1),
3 , CASE( MOD( DATEVALUE(EndDate__c) - DATEVALUE(startDate__c) ,7),1,2,2,2,3,2,4,3,5,4,6,5,1),
4 , CASE( MOD( DATEVALUE(EndDate__c) - DATEVALUE(startDate__c) ,7),1,1,2,1,3,2,4,3,5,4,6,5,1),
5 , CASE( MOD( DATEVALUE(EndDate__c) - DATEVALUE(startDate__c) ,7),1,0,2,1,3,2,4,3,5,4,6,5,0),
6 , CASE( MOD( DATEVALUE(EndDate__c) - DATEVALUE(startDate__c) ,7),1,1,2,2,3,3,4,4,5,5,6,5,0),
999)
+
(FLOOR(( DATEVALUE(EndDate__c) - DATEVALUE(startDate__c) )/7)*5)
Best Answer
If the value of your formula is consistently right, just negative, then maybe just add * -1 at the end of the formula. Or add a second field that gives the absolute value - i.e., ABS(Field__c)