After being first added to the panel, the plugin shows the time. But, after I open the Properties window and close it (changing nothing), the plugin becomes invisible on the panel as it shows nothing.
System: Debian testing, plugin version 0.6.2.
Sorry, this applies not to DateTime plugin but rather to the built-in panel plugin (showing as Clock in the list).
*** This bug has been marked as a duplicate of bug 11527 ***