I have my date formats configured under Tools>Preferences>Database>NLS, and I'm having no problems getting the format I want. A fellow colleague can do whatever they want on this screen, and they'll still get dates in a 'dd-mon-yy' format. The server has it's NLS_DATE_FORMAT set to MM/DD/RR. And. my co-worker is definitely not purposefully running an "alter session set nls_date_format = 'dd-mon-yy'".
Before I ask a colleague to blow away their profile and start over, is there some other location where this format could be set from within SQL Developer?
I checked, and he has nothing set for his startup script. I compared it to my setup, and mine has the word 'false' listed for a file location, which maybe got set a couple of days ago when I upgraded to version 3.2.20 though. My colleague is still on 3.2.09.
I also noticed on the NLS screen you can click on "Skip NLS Settings" button, although that option is unchecked on his screen.
I went ahead & just had him delete his \SQL Developer folder where preferences & settings are stored, forcing SQL Developer to start from scratch and rebuild of it, and now his NLS settings are working again.
The logon trigger was a good suggestion that I hadn't thought of, but since only one person was having this issue, I just took the easy way out.