Gregorian calendar:

Extending the Gregorian calendar backwards to dates preceding its official introduction produces a proleptic calendar, which should be used with some caution. For ordinary purposes, the dates of events occurring prior to 15 October 1582 are generally shown as they appeared in the Julian calendar, with the year starting on 1 January, and no conversion to their Gregorian equivalents. For example, the Battle of Agincourt is universally considered to have been fought on 25 October 1415 which is Saint Crispin's Day.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". Attempting to use Gregorian dates for earlier events  on the Julian calendar should be used with what action?
caution