December 26, 2 BC
December 26 of the year -1
Day 1 of week 1 of the year 0
The
year-of-era (
"y") is what you're most accustomed to: it starts at
January 1, 1 AD. The day before that is
December 31, 1 BC.
The
proleptic year (
"u") does the same for AD, except for BC it continues counting down: 0 for
1 BC, -1 for
2 BC, etc.
The
week-based-year (
"Y") works the same as the proleptic year, except it doesn't start counting years at the 1st of January, but at the first weekday of the first week of the year.
For example, if you start the week on Sunday (as the root locale does) then the
week-based-year 2016 started on
December 27, 2015.