Apple Numbers x-axis "Value Labels" can't be set to "Date & Time" or "Duration" I am graphing a two-column data set that looks like this:

1/1/1963    3,209,827,882
1/1/1964    3,281,201,306
1/1/1965    3,350,425,793


But when I try to set the Value Labels in either a Line Chart or a Scatter Plot to "Date &amp; Time" or "Duration" I find that they are greyed out.

Why would this be? In what situations is it possible to use these two options?
Make sure the source columns are formatted as the proper date/time format.