TL;DR When talking about date time patterns, Y, y and u all mean different things in Java, and if you don’t know that it’s painful when you discover it.

Admit it, if you hadn’t looked at the TL;DR above, what do you think

LocalDate.of(2019, 12, 30).format(DateTimeFormatter.ofPattern("MMMYY"));

would evaluate to?

Dec19, right? RIGHT?! RIGHT?!

NO! Apparently not! It evaluates to Dec20! Everything you thought you knew was a lie!

Why does this happen?

Because Y apparently means a “week-based” year, and the 30th December 2019 was the Monday of the first week of 2020, so Java thinks you want 2020.

I know, crazy!

What should be used instead is y or u. More info here:

• https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html
• https://stackoverflow.com/questions/29014225/what-is-the-difference-between-year-and-year-of-era

Makes you wonder how many things you’ve programmed by coincidence, doesn’t it?

Food for thought!