This query:
# Query birthdate of Vespasian
SELECT ?date ?precision ?rank
WHERE
{
wd:Q1419 p:P569 ?dob_statement.
?dob_statement wikibase:rank ?rank.
?dob_statement psv:P569 ?dob_value.
?dob_value wikibase:timePrecision ?precision.
?dob_value wikibase:timeValue ?date.
}gives three dates of birth for Q1419 (Vespasian). Compare with https://www.wikidata.org/wiki/Q1419#P569:
- The preferred date in the query is "15 November 0009", but item page says "17. november 9".
- The deprecated value in the query is "16 November 0009", but the item page says "18. november 9".
The API (wbgetclaims: https://www.wikidata.org/w/api.php?action=wbgetclaims&entity=Q1419&property=P569) agrees with the item page.
So it seems that WDQS gives dates that are two days earlier than they should be in this case.