Dates and Times
How you work with dates in Tableau depends on whether you’re using a relational or cube (multidimensional) data source. This section discusses the differences.
Dates in Relational Data Sources
For relational
data sources, dates and times are dimensions and are identified by the date 
or date-time 
icon. For example, the Order
Date and Ship Date dimensions
from an Excel data source are shown here.
When you place a relational date on a shelf, the field name is automatically modified to reflect the default date level. Tableau defines the default date level to be the level at which there are multiple instances. For example, if the date field includes multiple years, the default level is year. However, if the date field contains data for just one year but includes multiple months, then the default level is month.
If you don’t want Tableau to select a date level automatically and prefer a continuous field, you can right-click (control-click on Mac) the field in the Data pane and select Convert to Continuous. The dimension then turns green in the Data pane; now when you use that dimension in a view, it will be continuous. You can easily revert by selecting Convert to Discrete from the field’s context menu in the Data pane. You can also convert a field in the view to continuous while it is on a shelf by selecting Continuous on its context menu (right-click (control-click on Mac) the field). The field on the shelf turns green but the field in the Data pane is still discrete.
Dates in Cube (Multidimensional) Data Sources
In Tableau Desktop, cube (multidimensional) data sources are supported only in Windows.
For cube data sources, dates dimensions are usually organized into hierarchies that contain levels such as year, quarter, and month. In addition, some multidimensional data sources have time intelligence enabled, which makes it possible to look at data levels different ways, such as Months by Quarter, Weekends, and so on These levels are represented as attributes of the hierarchy. Hierarchies and attributes are defined when the cube is created and you can’t modify them in Tableau. For example, the Year dimension from an Oracle Essbase data source is shown here.
When you place a multidimensional date on a shelf, the field is treated like any other dimension. For example, you can drill down, drill up, and so on.