All files in Finder show 'Date Modified' with correct days but always the same hour: 19:08 When I open a Finder window, all the file timestamps show the same hour, 19:08. The dates change, but the hour stays constant. This is also the case for time/dates shown in the Console.

The Clock on the top right shows the correct time.

A datecommand in Terminal shows the correct time.

I have zapped my NVRAM and also replaced the on-board battery.

I am using a MacPro 1,1 with Mac OS X 10.7.5
This behaviour can be seem if the date is customised in the System Preferences / Language and Text pane.

To fix this - go to System Preferences, then Languages and Text.
Click 'Formats' tab, then the 'customise' button next to 'Times'.

You should see all the elements of the proposed time formats as having light blue backgrounds.  If you see any text on a plain background this is literal text rather than a placeholder.   

On previous screen, the 'Formats' tab. Change 'Region' dropdown at top of page from 'Custom' to an appropriate region for you to give you a sensible set of defaults.

To fix and keep your customised settings, delete the plain text, and then drag and drop from blue placeholders to set your time formats.  Drag the hour placeholder, type a colon and drag the minute placeholder.