We use the proleptic Gregorian calendar internally (via Date::Calc), which is a sensible choice. In display, however, it can lead to confusion where other sources use the Julian calendar; which they usually do for events from before the Gregorian calendar was first introduced (1582-10-15). In many countries, the Gregorian calendar was not adopted until much later (1923 in Greece).

Therefore, we should display dates before 1582-10-15 in the Julian calendar, with a note (e.g., "1492-10-12 O.S." with an explaining tooltip or link); and dates from the transition period in both calendars, with the Julian calendar representation in a smaller type or similar (see how Wikipedia does it for J.S. Bach's birthdate: Johann Sebastian Bach).

Ideally, this would be complemented in the editing interface by a date field that can do on-the-fly conversion from the Julian, Hebrew, etc. calendars.

Suitable module: Date::Convert::Julian