Palindrome Day 20200202

By the way

The very first palindromic date in ISO YYYYMMDD format after 1000-01-01 (Jan 01 1000) was 1001-10-01 (Oct 01 1001).

2020-02-02 is the 47^th palindromic date in the YYYYMMDD format after the reference date 1000-01-01. In total, 372579 days have passed since then. The 331^st and very last such date before the year 10000 is 9290-09-29 (Sep 29 9290). This will be the 3028132^nd day after 1000-01-01 and therefore is still 2655553 days away (after 2020-02-02).

It is a little bit different in the DDMMYYYY format. The following applies here: 02.02.2020 is the 67^th palindromic date after the reference date 01.01.1000. The last possible symmetrical date before the year numbers become five digits long one is 29.09.9092 which is the 335^th such date in the list. It’s just the 2955814^th day after 01.01.1000.

Hint: Because of the Gregorian Calendar Reform of 1582, the numbers of days after 1001-01-01 are fictitious, strictly spoken. They are valid only then, if we adopt the reform for dates before 1582, subsequently. To get the real number of days after ‘Jan 1 1000’ for years after 1582 subtract 5 from each term representing a date after 1582. This applies to all palindromic dates after the 43^rd in YYYYMMDD format, and after the 61^st in DDMMYYYY format. The value 5 comes from the 10 skipped “Gregorian” dates between 1582-10-04 (Oct 4 1582) and 1582-10-15 (Oct 15 1582) minus the 5 additional Julian leap days in the years 1000, 1100, 1300, 1400 and 1500 which are not Gregorian leap years.

from Hacker News https://ift.tt/2S92nBM