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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](https://date-fns.org/docs/Change-Log).
+
+### Camel case naming schema
+
+Function submodules now use camelCase naming schema:
+
+```javascript
+// 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](https://date-fns.org/docs/Unicode-Tokens) 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](https://blog.date-fns.org/post/we-cut-date-fns-v2-minimal-build-size-down-to-300-bytes-and-now-its-the-smallest-date-library-18f2nvh2z0yal) for more details.
+
+```javascript
+// 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   |
+
+Notes:
+
+- 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](https://github.com/date-fns/date-fns/pull/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](https://github.com/date-fns/date-fns/pull/460) and
+[#765](https://github.com/date-fns/date-fns/pull/765) for exact behavior.
+
+### `null`
+
+`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](https://github.com/date-fns/date-fns/issues/537) for the reasoning.
+
+### `RangeError`
+
+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.
+
+### `TypeError`
+
+All functions now check if the passed number of arguments is less
+than the number of required arguments and throw `TypeError` exception if so.
+
+### UMD/CDN
+
+The Bower & UMD/CDN package versions are no longer supported.
+
+### New locale format
+
+See [docs/Locale](https://date-fns.org/docs/Locale).
+
+Locales renamed:
+
+- `en` → `en-US`
+- `zh_cn` → `zh-CN`
+- `zh_tw` → `zh-TW`
+
+```javascript
+// Before v2.0.0
+import locale from 'date-fns/locale/zh_cn'
+
+// v2.0.0 onward
+import locale from 'date-fns/locale/zh-CN'
+```
+
+[unicode tokens]: https://www.unicode.org/reports/tr35/tr35-dates.html#Date_Field_Symbol_Table