Andres, If you want to replicate the Excel method, just create a calculated field with your date field.
Datediff('day','01/01/1900',your datefield). (excel does this logic)
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Just to reference the issue. There is a two days difference between the DATEDIFF calculated value in Tableau and what Excel presents for a date.
TABLEAU 2/28/2013 == 41331 datediff('day',#1/1/1900#,#2/28/2013#)
EXCEL 2/28/2013 == 41333