I have the following object of type java.util.Date (Java 8):
So basically 1 January 1235 in CET format. But the moment I cast this with Instant:
IntelliJ show me that a whole week has been added! Instant uses UTC format, which explains why one hour is subtracted (one hour difference between CET and UTC). But where does the week come from?
That's because most of humanity was using Julian calendar in 1235, and has switched to Gregorian since.
Old style Java Dates do a best effort to account for this when it can deduce for a Locale when it did the switch from Julian to Calendar.
When these switches were made, the date in these countries switched to a different place on the calendar, making it look like the date just skipped a bunch of days.
The newer java.time.* system ignores the historic existence of Julian calendars and is ill-suited to represent time gaps in historic dates.
