If the start date is exclusive of the calc then you should modify start date (or end date) accordingly given NETWORKDAYS is inclusive by nature:

=NETWORKDAYS(A2+1,A1)/5

would return 10.2 rather than 10.4

Prior to XL2007 the use of NETWORKDAYS requires activation of the Analysis ToolPak Add-In (tools -> addins)

note: delimiters differ pending your locale - this in unspecified - modify as required