I have an old copy of my PalmOS calendar, but no longer have the device. How can I import these into Thunderbird/Lighting or convert them to .ics?
The short answer is that you cannot, at least not directly. Palm's Desktop application only supports exporting in their own Date Book Archive (.dba). Thunderbird/Lighting uses the standard (.ics - RFC 2445/RFC 5545) format.
palm2ical is a tool that runs on Windows, Mac, Linux. It's self contained.
The next problem was how to get the DatebookDB.PDB or CalendarDB-PDat.PDB file if you no longer have your PDA.
The page talks about:
Extracting the PDB file from your Palm
Once you have the database file (.pdb), you can run the utility:
There are also other tools listed under "Similar Programs" although I have not tested them.