Tableau changed how it parses dates in version 10.2 to handle a wider variety of date formats when we change the data type to date, there are a few cases where this can break existing data type conversions and this is one of them.
The workaround is to use DATE([yourDateField]) if the yourDateField is formatted as a number in Excel, if it's formatted as text in Excel then use DATE(INT([yourDateField])).
I'd also suggest submitting this to Tableau support so they can log it and potentially correct for this problem.
Thank you Jonathan.
I understand better the problem now.
However the DATE function is also inconsistent...
On the date 43100, Excel gives me 12/31/2017, Tableau gives me 12/30/1899 when using the data type and the calculated field DATE([yourDateField]) gives me 1/2/2018 ....
I'll submit a ticket.
EDIT: It seems the calculated field DATE([yourDateField])-2 gives me correct results. But I did only some random check...That will do the time to get the fix