[whpalmer4]

Have you considered using OmniFocus? It tracks that data automatically, and has (just a few) other features, too :-)

Alternatively, it would be possible to code up a pair of Applescripts which you could add to the OmniOutliner toolbar, one which prompts you for some text, creates the action and stamps the creation date/time in the notes field or a separate column, and another which completes the selected task and stamps the completion date/time in the corresponding place.

Yet another option: add the date yourself, with the keyboard shortcuts. cmd-| is the one I always use, which gets you a short date/time formatted something like

6/1/09 8:38:12 AM

at the insertion point. cmd-/ gets you just the date. They seem to work in a date column as well as a rich text field.

There's nothing in the file format that suggests that OmniOutliner keeps track in any way precisely when you added or modified a row.