Sometimes Tableau does not identify a date field as a date data type. Tableau may initially identify your date field as a string data type for example. You will be able to tell if Tableau identified the field as a date data type if you see the calendar icon next to the name of the field.
If you need to change the data type of your field then you can click on the data type icon next to the name of the field and select date in the menu. Depending on the format of your data, you may need to use DATEPARSE instead to convert the field. Please see this Tableau post for more information about how to convert a field to a date field.