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.
```js // ❌ 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:
```js 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)' ```