Please see here: https://forums.bitfire.at/post/7538
Please make sure that your server doesn’t serve any VCard with the questionable REV time format (i.e. apply >the OwnCloud Contacts fix and then correct invalid VCards).
After that, synchronize all contacts again.
Step 2: To do so, unselect the address books in DAVdroid, force sync, wait a bit until the local contacts are empty and sync again.
Although there is not that single solution for all occurancies, most of the systems are able to handle custom fields. This should allow to add such a field to DAVx5 too. I think it would always be better to cover at least 70-80% of the systems instead of 0 %. I just had a look on it and checked how other systems are handling a death day field. Here are my findings:
Nextcloud 15.0.2/Contacts 3.0.3 and ownCloud 10.0.10/Contacts 1.5.5 both handle the custom field Todestag in this way:
BEGIN:VCARD
VERSION:3.0
PRODID:-//Sabre//Sabre VObject 4.1.6//EN
UID:2ba8a291-5cc9-4865-a9c4-4d053a0ab080
FN:Dean Death
BDAY:19500101T000015
DEATHDATE:20181231T000000
END:VCARD
Android 8.0 contact sync’ed using the ContactSync app shows the custom field Todestag correctly:
Thunderbird 60.5.0/Cardbook add-on 33.9 shows the custom field label Todestag correctly:
IOS 12.1 is also able to handle custom labels although the vcf syntax differs a bit:
BEGIN:VCARD
VERSION:3.0
PRODID:-//Apple Inc.//iPhone OS 12.1//EN
N:Dean Death;;;;
FN: Dean Death
BDAY:1980-01-01
item1.X-ABDATE;type=pref:2000-04-30
item1.X-ABLabel:Todestag
END:VCARD
