Why has 1582/10 (yyyy/mm) only 21 days?

Question

SQL> edit
Escrito file afiedt.buf
  1  with
  2  yyyy as (select to_char(rownum-4713,'s0009') yyyy from dual connect by rownum <= 9999+4713)
  3  ,mm as (select to_char(rownum,'09') mm from dual connect by rownum <= 12)
  4  select last_day(to_date(yyyy||mm,'syyyymm'))-to_date(yyyy||mm,'syyyymm')+1 as days
  5        ,to_char(min(to_date(yyyy||mm,'syyyymm')),'syyyy/mm') as min_yyyymm
  6        ,to_char(max(to_date(yyyy||mm,'syyyymm')),'syyyy/mm') as max_yyyymm
  7  from yyyy,mm
  8  where yyyy.yyyy != 0
  9  group by last_day(to_date(yyyy||mm,'syyyymm'))-to_date(yyyy||mm,'syyyymm')+1
10* order by days
SQL> /
      DAYS MIN_YYYY MAX_YYYY
        21  1582/10  1582/10
        28 -4712/02  9999/02
        29 -4708/02  9996/02
        30 -4712/04  9999/11
        31 -4712/01  9999/12

Answer 1

If Oracle supports old calendar of each countriesnope. there is only one gregorian calendar. some countries just took a while to get around to using it. it'd be like asking if they support the julian calendar, which they don't (don't confuse the julian calendar with "julian days").
and the only way you'd have a problem now is if you were storing historical dates, and something happened in britian on Oct 10th, 1582. Because while britian recognized that date, pope gregory did not, and oracle will convert it to Oct 15.
side note, it made things somewhat confusing for people born in those times. most of America's founding fathers were alive when Britian switched to Gregorian, so all of a sudden, ages, contract durations, and prison sentences had to be adjusted.