Recently, a timestamp with calendarmodel https://www.wikidata.org/wiki/Q12138 has been introduced into
Wikidata: https://www.wikidata.org/w/index.php?title=Q105958428&oldid=2004936527

I think this should not be possible and there should be checks to only allow values Q1985727 and Q1985786 as it is documented here: https://www.wikidata.org/wiki/Help:Dates#Time_datatype