I fixed my issue. Some searching led me to this:
https://github.com/MichMich/MagicMirror/issues/791#issuecomment-289306407
No dates before 1970 are accepted - but in my birthday calendar I have created birthdays e.g. in 1946…
Thank you @amcolash

This is how you can check if this is why your calendar is not working:
ssh to your pi

cd /home/pi/MagicMirror
DIAPLAY=:0 npm start dev

If you see this in the output, you have dates in your calendar before 1970:

Whoops! There was an uncaught exception...
Error: dates lower than Thu Jan 01 1970 00:00:00 GMT+0100 (CET) are not supported
    at Object.toOrdinal (/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js:105:15) 
    at Iterinfo.rebuild (/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js:1409:35)
    at RRule._iter (/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js:978:10)
    at RRule.between (/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js:794:21)
    at /home/pi/MagicMirror/modules/default/calendar/calendarfetcher.js:122:24
    at Request._callback (/home/pi/MagicMirror/modules/default/calendar/vendor/ical.js/node-ical.js:11:5)
    at Request.self.callback (/home/pi/MagicMirror/node_modules/request/request.js:188:22)
    at emitTwo (events.js:106:13)
    at Request.emit (events.js:191:7)
    at Request. (/home/pi/MagicMirror/node_modules/request/request.js:1171:10)
MagicMirror will not quit, but it might be a good idea to check why this happened. Maybe no internet connection?
If you think this really is an issue, please open an issue on GitHub: https://github.com/MichMich/MagicMirror/issues

What I did to fix my birthday calendar: Edit this file

/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js

Go to line 56 or search for the first occurance of “ORDINAL_BASE” and change this line:

ORDINAL_BASE: new Date(1970, 0, 1),

set it to a lower year like 1900.
Then restart your magic mirror (pm2 restart mm)

And it’s (hopefully) GREAT AGAIN :-D :clap:

@Mitch1138
You have changed your ORDINAL_BASE to 1900, but it seems as if you have dates even before 1900 in your calendar as it tells you in line 2 of the exception:

Error: dates lower than Mon Jan 01 1900 00:00:00 GMT-0500 (EST) are not supported

What kind of calendar is it? Try to change the year to something even lower or even 0. Otherwise you can perhaps guess based on your calendar which entry might cause this error and change that recurring date…

@fuzzylogic725
I had the same issue and fixed it:
https://forum.magicmirror.builders/post/19902

The hint is in line two of the logs: dates before 1970 are not permitted.

I fixed my issue. Some searching led me to this:
https://github.com/MichMich/MagicMirror/issues/791#issuecomment-289306407
No dates before 1970 are accepted - but in my birthday calendar I have created birthdays e.g. in 1946…
Thank you @amcolash

This is how you can check if this is why your calendar is not working:
ssh to your pi

cd /home/pi/MagicMirror
DIAPLAY=:0 npm start dev

If you see this in the output, you have dates in your calendar before 1970:

Whoops! There was an uncaught exception...
Error: dates lower than Thu Jan 01 1970 00:00:00 GMT+0100 (CET) are not supported
    at Object.toOrdinal (/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js:105:15) 
    at Iterinfo.rebuild (/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js:1409:35)
    at RRule._iter (/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js:978:10)
    at RRule.between (/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js:794:21)
    at /home/pi/MagicMirror/modules/default/calendar/calendarfetcher.js:122:24
    at Request._callback (/home/pi/MagicMirror/modules/default/calendar/vendor/ical.js/node-ical.js:11:5)
    at Request.self.callback (/home/pi/MagicMirror/node_modules/request/request.js:188:22)
    at emitTwo (events.js:106:13)
    at Request.emit (events.js:191:7)
    at Request. (/home/pi/MagicMirror/node_modules/request/request.js:1171:10)
MagicMirror will not quit, but it might be a good idea to check why this happened. Maybe no internet connection?
If you think this really is an issue, please open an issue on GitHub: https://github.com/MichMich/MagicMirror/issues

What I did to fix my birthday calendar: Edit this file

/home/pi/MagicMirror/node_modules/rrule-alt/lib/rrule.js

Go to line 56 or search for the first occurance of “ORDINAL_BASE” and change this line:

ORDINAL_BASE: new Date(1970, 0, 1),

set it to a lower year like 1900.
Then restart your magic mirror (pm2 restart mm)

And it’s (hopefully) GREAT AGAIN :-D :clap:

Hi,
what version are you using? This might be an issue that came with the update to version 2.1.1, check these threads:
https://github.com/MichMich/MagicMirror/issues/839
https://forum.magicmirror.builders/topic/2221/after-upgrade-v2-1-1-multiple-calendars-not-displaying
https://forum.magicmirror.builders/topic/2208/calendar-shows-no-entries-after-mm-update
I am experiencing troubles with one of my owncloud calendars.

Hi,
I am experiencing the same issue since the update to the current MM version 2.1.1 . I should have checked for issues beforehand :-/
EDIT: I have all my calendars on my owncloud on the same LAN. All of them work fine except a calendar with all birthdays.
Since I have no time to troubleshoot myself for the next month, I will have to wait for an official fix…
Let’s make MM great again ;-)