Calendar buttons are found in the Prescription window and elsewhere.
Clicking these buttons opens a calendar dialog sheet.
The Calendar Sheet 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.
The standard OSX controls are duplicated in this sheet and are supplemented with graphical user elements as well. You can advance the month and year using the arrows in the calendar header, 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.
The Current Time button copies the current date and time from the computer's system clock.
Click OK to exit the dialog.