The Excel number for a modern date is most easily calculated as the number of days since 12/30/1899 on the Gregorian calendar.
Attached please find a v10.0 workflow that converts a double (Excel date) to a Datetime in Alteryx.
It uses this function in a Formula Tool:
DateTimeAdd([Datebase],[Field1],"days")
where Datebase = a datetime value set to "1899-12-30 00:00:00"
and Field1 is double value = 42397