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v2 Upgrade Guide

Common changes

This page covers a few of the most common problems people face when updating from v1 to v2. For a more detailed list of changes, look at the change log for version 2.0.0.

Camel case naming schema

Function submodules now use camelCase naming schema:

// Before v2.0.0
import differenceInCalendarISOYears from 'date-fns/difference_in_calendar_iso_years'

// v2.0.0 onward
import differenceInCalendarISOYears from 'date-fns/differenceInCalendarISOYears'

New formatting tokens

Starting with v2 format and parse uses Unicode tokens.

See Unicode Tokens doc for more details.

String arguments

Functions now don't accept strings as date arguments. Strings should be parsed using parseISO (ISO 8601) or parse.

See this post for more details.

// Before v2.0.0
addDays('2016-01-01', 1)

// v2.0.0 onward
addDays(parseISO('2016-01-01'), 1)

Arguments conversion

All functions now implicitly convert arguments by following rules:

date number string boolean
0 new Date(0) 0 '0' false
'0' Invalid Date 0 '0' false
1 new Date(1) 1 '1' true
'1' Invalid Date 1 '1' true
true Invalid Date NaN 'true' true
false Invalid Date NaN 'false' false
null Invalid Date NaN 'null' false
undefined Invalid Date NaN 'undefined' false
NaN Invalid Date NaN 'NaN' false


  • as before, arguments expected to be Date are converted to Date using date-fns' toDate function;
  • arguments expected to be numbers are converted to integer numbers using our custom toInteger implementation (see #765);
  • arguments expected to be strings are converted to strings using JavaScript's String function;
  • arguments expected to be booleans are converted to boolean using JavaScript's Boolean function.

null and undefined passed to optional arguments (i.e. properties of options argument) are ignored as if no argument was passed.

If any argument is invalid (i.e. NaN for numbers and Invalid Date for dates), an invalid value will be returned:

  • false for functions that return booleans (expect isValid);
  • Invalid Date for functions that return dates;
  • NaN for functions that return numbers;
  • and String('Invalid Date') for functions that return strings.

See tests and PRs #460 and #765 for exact behavior.


null now is not a valid date. isValid(null) returns false; toDate(null) returns an invalid date. Since toDate is used internally by all the functions, operations over null will also return an invalid date. See #537 for the reasoning.


Functions now throw RangeError if optional values passed to options are not undefined or have expected values. This change is introduced for consistency with ECMAScript standard library which does the same.


All functions now check if the passed number of arguments is less than the number of required arguments and throw TypeError exception if so.


The Bower & UMD/CDN package versions are no longer supported.

New locale format

See docs/Locale.

Locales renamed:

  • enen-US
  • zh_cnzh-CN
  • zh_twzh-TW
// Before v2.0.0
import locale from 'date-fns/locale/zh_cn'

// v2.0.0 onward
import locale from 'date-fns/locale/zh-CN'