Timezone handling is poor to non-existent.

Date Popup Authored works correctly when the user's timezone, the site's timezone, and the server's timezone all match. However, when the site timezone and the user timezone differ, Date Popup starts play fast and loose with the time entered.

For example, let's say example.com's timezone is America/New_York (GMT-4), and my timezone is America/Los_Angeles (GMT-7). I create a node and enter "January 1, 2010 - 12:00pm" in the Date Popup Authored field. When I save the node, the time is converted to and saved using the site's timezone.

Now, the node's timezone is saved as "January 1, 2010 - 3:00pm". When I edit the node, Date Popup Authored is populated with "January 1, 2010 - 3:00pm" (the site's timezone). When I click save without changing anything, the node's time jumps into the future again to "January 1, 2010 - 6:00pm".

The solution is to use Date API to get the user's timezone and save the time in the exact format the original Authored field expects. This should solve any problem with dates not appearing or saving correctly. A fix is forthcoming.