Can someone recommend a calendar program?

[Tyrannocaster]

it sounds like Evolution would be fine, except that I have three different operating systems and I chose Thunderbird partly because I could export its messages to the others if a I had Thunderbird on them. I got trapped because I used Opera for mail for about 15 years on the Mac (and I still like it) but you can not export the emails, even though supposedly you can. And even if you do, you end up with something unusable; it's a nightmare from a compatibility standpoint.

Can you export Evolution's emails so that Thunderbird can read them?