Set Custom Date Format in SQL Developer.

You can change this in preferences:

1. From Oracle SQL Developer's menu go to: Tools > Preferences.
2. From the Preferences dialog, select Database > NLS from the left panel.
3. From the list of NLS parameters, enter DD-MON-RR HH24:MI:SS into the Date Format field.
4. Save and close the dialog, done!

Here is a screenshot: