== one date, two letters ==
date                    = 2026-12-29 (TUESDAY)
dd-MM-yyyy              = 29-12-2026
dd-MM-YYYY              = 29-12-2027

== the same YYYY pattern, four locales, three dates ==
date        en_US     en_GB     fr_FR     en_IN     
2026-12-28  2027      2026      2026      2027      
2026-12-29  2027      2026      2026      2027      
2027-01-01  2027      2026      2026      2027      

== why: each locale defines what a week is (WeekFields) ==
en_US  WeekFields[SUNDAY,1]   week-based-year of 2026-12-29 = 2027, week number = 1
en_GB  WeekFields[MONDAY,4]   week-based-year of 2026-12-29 = 2026, week number = 53
fr_FR  WeekFields[MONDAY,4]   week-based-year of 2026-12-29 = 2026, week number = 53
en_IN  WeekFields[SUNDAY,1]   week-based-year of 2026-12-29 = 2027, week number = 1

== how often is YYYY wrong? every day of 2020-2030, compared with yyyy ==
en_US   36 wrong days out of 4018
en_GB   19 wrong days out of 4018
fr_FR   19 wrong days out of 4018
en_IN   36 wrong days out of 4018

== the days that are wrong in the US locale ==
2024/2025: 2024-12-29->2025, 2024-12-30->2025, 2024-12-31->2025
2025/2026: 2025-12-28->2026, 2025-12-29->2026, 2025-12-30->2026, 2025-12-31->2026
2026/2027: 2026-12-27->2027, 2026-12-28->2027, 2026-12-29->2027, 2026-12-30->2027, 2026-12-31->2027
2027/2028: 2027-12-26->2028, 2027-12-27->2028, 2027-12-28->2028, 2027-12-29->2028, 2027-12-30->2028, 2027-12-31->2028
2028/2029: 2028-12-31->2029

== yyyy vs uuuu: same for modern dates, different for the era ==
date = -0043-03-15
yyyy = 0044
uuuu = -0043
yyyy G = 0044 BC