Mac OS X 10.5 (Leopard) gives you extensive options for
setting and displaying dates and times exactly as you want. It
only requires setting a few preferences. To begin, open the Mac
OS X 10.5 System Preferences and select the International
preferences pane. Next, select the Formats tab.

To set dates, click the Customize... button on the top
section of the Formats pane (above Calendar). Another pane
will open with several options for customizing dates.

You can choose to display the date in four lengths:
To set the length for the display of dates, click the button
next to Show and select the option you want from the
drop-down menu. Note that you can change any of the formats by
adding and subtracting items from the lower part of the pane so
it appears that the Show option is merely a shortcut for
setting the display options.
To add an item from the lower part of the pane, drag it to the
text box above the Date Elements options and under the
Show option. To remove an item, highlight it and then
press the Delete key. You can also change some of the
items you add by clicking the arrow at the end of an item and
selecting a variation of the item from the drop-down menu that
appears (for example, you can change the year from 09 to
2009).

When you have finished setting date options, click the OK
button to return to the Formats pane.
Your options for setting times are similar to those for dates.
To access the options, click the Customize... button in
the Times section of the Formats pane. The Times Customization
pane will open with several options for customizing times.

Like the Dates option, you can choose to display the time in four
lengths:
To set the length for the display of times, click the button
next to Show and select the option you want from the
drop-down menu. Note that you can change any of the formats by
adding and subtracting items from the lower part of the pane so
it appears that the Show option is merely a shortcut for
setting the display options.
To add an item from the lower part of the pane, drag it to the
text box above the Time Elements options and under the
Show option. To remove an item, highlight it and then
press the Delete key. You can also change some of the
items you add by clicking the arrow at the end of an item and
selecting a variation of the item from the drop-down menu that
appears (for example, you can change EST to Eastern Standard
Time).

When you have finished setting time options, click the OK
button to return to the Formats pane.