Starting with v2, format and parse use Unicode tokens.

The tokens are different from Moment.js and other libraries that opted to use custom formatting rules. While usage of a standard ensures compatibility and the future of the library, it causes confusion that this document intends to resolve.

There are 4 tokens that cause most of the confusion:

• D and DD that represent the day of a year (1, 2, ..., 365, 366) are often confused with d and dd that represent the day of a month (1, 2, ..., 31).

• YY and YYYY that represent the local week-numbering year (44, 01, 00, 17) are often confused with yy and yyyy that represent the calendar year.

// ❌ Wrong!
format(new Date(), "YYYY-MM-DD");
//=> 2018-10-283

// ✅ Correct
format(new Date(), "yyyy-MM-dd");
//=> 2018-10-10

// ❌ Wrong!
parse("11.02.87", "D.MM.YY", new Date()).toString();
//=> 'Sat Jan 11 1986 00:00:00 GMT+0200 (EET)'

// ✅ Correct
parse("11.02.87", "d.MM.yy", new Date()).toString();
//=> 'Wed Feb 11 1987 00:00:00 GMT+0200 (EET)'

To help with the issue, format and parse functions won't accept these tokens without useAdditionalDayOfYearTokens option for D and DD and useAdditionalWeekYearTokens options for YY and YYYY:

format(new Date(), "D", { useAdditionalDayOfYearTokens: true });
//=> '283'

parse("365+1987", "DD+YYYY", new Date(), {
  useAdditionalDayOfYearTokens: true,
  useAdditionalWeekYearTokens: true,
}).toString();
//=> 'Wed Dec 31 1986 00:00:00 GMT+0200 (EET)'