try that:

[VBA]
Option Explicit
Sub TEST()
Dim str_Date1 As Date
Dim str_Date2 As Date
str_Date1 = "28/03/2008"
str_Date2 = "25/07/2009"
MsgBox Application.WorksheetFunction.Days360(str_Date1, str_Date2)
End Sub
[/VBA]

or:

[VBA]
Option Explicit
Sub TEST()
Dim str_Date1 As Date
Dim str_Date2 As Date
str_Date1 = Range("A1").Value
str_Date2 = Range("A2").Value
MsgBox Application.WorksheetFunction.Days360(str_Date1, str_Date2)
End Sub
[/VBA]