Always wondered why my Android calendar on CyanogenMod13 (Android 6.0) doesn't show up some birthday events from a few but not all contacts. After debugging i noticed that i've set a Birthday of 1900 for Contacts where i don't know the birth year.
As the Birthday evens where showing up in Thunderbird as well as in the oC internal calendar i posted a bug at the DAVdroid forums:
The thread led to an issue in 32bit integers on such 32bit devices like my smartphone. @davdroid did a really great debugging and reported that issue to the Android bugtracker:
If you're affected by the same issue the workaround is for now to set a birthday year to e.g. 1902 / 2000 or similar.