Apple Numbers '09 - Manual (Page 73)

Chapter 3 Using Tables 73
Table categories are created based on unique values in the chosen column.
Category row
Category value column
If the category value column contains dates, you can choose a date unit from the
second pop-up menu.
Category column that contains
dates
Choose a date unit.
The date unit you select controls how the rows are categorized as well as how the
category is identified in the category row.
Category rows
display years because
“years” was selected in the
Reorganize window.