Ruby’s Date::ITALY constant (2299161) is the Julian day number marking Italy’s 1582 switch from the Julian to Gregorian calendar.
Key Takeaways
Date::ITALY, Date::ENGLAND, Date::GREGORIAN, and Date::JULIAN are Ruby constants encoding Julian day numbers for calendar reform transitions.
Ruby raises Date::Error for dates in the 10-day gap (Oct 5-14, 1582) and silently skips them when iterating.
Date::JULIAN is Float::INFINITY (never switch to Gregorian); Date::GREGORIAN is -Float::INFINITY (always Gregorian).
The optional start argument in Date.new lets you model any country’s switchover, including Russia’s 1918 adoption via Julian day 2421639.
Julian day numbers count continuously from noon UTC on Jan 1, 4713 BC, a tricyclic synchronization of 28-year solar, 19-year lunar, and 15-year indiction cycles.
Hacker News Comment Review
Commenters noted calendar adoption was not uniform even within countries: cities and religious communities (Protestant vs. Catholic) could operate on different calendars simultaneously, creating ambiguity about which date to use depending on the recipient of correspondence.
Julian dates are still used in encrypted radio systems and astronomy precisely because they skip no days, functioning like a pre-Unix epoch continuous counter without timezone handling.
The fuzzed-input test bug where one random seed exposed date-handling errors illustrates that Ruby’s calendar-gap behavior can surface as surprising runtime failures far from obvious call sites.
Notable Comments
@cdot2: Julian dates are used in encrypted radio timing systems; astronomers adopted them because the continuous count avoids gaps that would break long-term cycle tracking.
