Welcome to the forums.
Try the following formula in a calculated field to convert your Excel year into a date format that Tableau will recognize.
Date ("1/1/" + str([<your current year field>]))
This will give each year a month and day of January 1. If you want a different month/day combination represented (perhaps 12/31), just swap out "1/1/" with what you'd like to see.
Hope this helps,