The Calendar button opens the calendar dialog.

The Calendar Dialog provides an alternative method of setting dates and times compared to the OSX standard date & time controls found in the Prescription and some other windows and dialogs. The OSX controls are duplicated in this dialog. In addition, you can set the month from a popup menu, click on a date in the calendar to select that date, and you can click on the clock to set a time. Clicking on the clock again exchanges AM and PM context. The Current Time button copies the current date and time from the computer's system clock. Click OK to exit the dialog.