The Day of Calendar function is supported, returning an integer 1-n, the number of Julian days since 1/1/1900. When dragging a Day of Calendar into a variable, the following tree element is created.
Variable Creation > Input > Variables: SQL Elements pane - Calendar > Day Of Calendar

Columns and/or other expressions that resolve to a date can be moved over the (empty) branch of the tree.
There are no special properties for the Day of Calendar function.