Debian Icedove - how to transfer calendar (iceowl) and contacts from one computer to another

I'm installing a new computer; I moved from Jessie to Stretch on that occasion; in both cases using Icedove with the Iceowl calendar extension. IMAP e-mail setup was zero work, but now I need to get my contacts and the calendar entries from the old to the new computer. How would I do that?