Custom date time format in Oracle SQL Developer?
From Oracle SQL Developer's menu go to Tools -- Preferences.
From the Preferences dialog, select Database -- NLS from the left panel.
From the list of NLS parameters, enter
DD-MON-RR HH24:MI:SS
or for 24-Hour,
DD-MON-YY HH24:MI:SS
Or just for an active session use below instead.
alter SESSION set NLS_DATE_FORMAT = 'required_date_format'
i.e.
alter SESSION set NLS_DATE_FORMAT = 'DD-MM-YYYY HH24:MI:SS'
into the Date Format field.
Save and close the dialog.