Date-fns: YYYY should be yyyy and DD should be dd

[tkrotoff]

The tokens accepted by date-fns are different with what most languages and libraries support.

I focus here on YYYY and DD.

Comparable:

• AngularJS: yyyy dd instead of YYYY DD
• Qt: yyyy dd instead of YYYY DD
• C#: yyyy dd instead of YYYY DD
• Java: yyyy dd instead of YYYY DD
• Objective-C: yyyy dd instead of YYYY DD
• Dart: yyyy dd instead of YYYY DD

They all follow the specifications from the Unicode Consortium CLDR: http://www.unicode.org/reports/tr35/tr35-31/tr35-dates.html#Date_Format_Patterns

Not comparable:

• Python: % based
• PHP: no yyyy nor YYYY
• C/C++: % based
• Ruby: % based

Interpreted languages are mostly based on C function strftime() (so they are % based) since the interpreters are written in C. I guess strftime() predates the Unicode standard by several decades, that's why.

Edit: somewhat related to #351

[leshakoss]

Merged, going to be fixed with the next release

[mesqueeb]

@leshakoss Is this a breaking change? Should all localization files also be updated for this?

[leshakoss]

@mesqueeb yes it is and yes they should. I've updated the contribution guide some time ago. Here is the list of locales to be updated: https://github.com/date-fns/date-fns/blob/master/outdatedLocales.json