If the user interface is invoked by typing, for example, "Galileo Galilei" in the search box and clicking on the obvious item, the date of birth (P569) and the date of death (P570) may be displayed incorrectly. In every case the date stored in Wikidata, which is always a Gregorian calendar date (proleptic, if necessary) will be displayed. But sometimes (for example, if the date is before the introduction of the Gregorian calendar on 15 October 1582) the date has the word "Julian" next to it. This is the case for Galileo Galilei's birth date of 25 February 1564 Gregorian, which is displayed as "25 February 1564 //Julian//" (with the word "Julian" superscripted).
This error has caused numerous incorrect dates to be inserted into Wikidata by editors who did not understand what was being shown to them.