zloirock/core-js

ES6 Date parsing

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#63 opened on May 2, 2015

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enhancementeshelp wanted

Description

The behavior or parsing a date or date+time string to a Date object (using the new Date(string) constructor, or Date.parse(string) function) is changed in ES6. AFAIK, this isn't currently handled by the core-js ES6 polyfills. Should it be? My understanding is that this is a dependency for libraries such as Babel. (originally filed as babel/babel#1418)

Behavior

ES6 §20.3.1.16

If the time zone offset is absent, the date-time is interpreted as a local time.

ES5.1 §15.9.1.15:

The value of an absent time zone offset is "Z".

To get ES6 behavior out of ES5, the string would need to be mutated to be interpreted in local time. The polyfill would have to be careful only to do this for the specific use cases affected.

ES6 ES5
new Date("2014-12-31") new Date("2014/12/31")
new Date("2014-12-31T01:23:45") new Date("2014/12/31 01:23:45")
new Date("2014-12-31T01:23:45Z") no change
new Date("2014-12-31T01:23:45-05:00") no change

Note, the hyphens are replaced with slashes, but still kept in year/month/day order to prevent internationalization ambiguities. Also note the T removed. Also note that ECMAScript only defines the ISO8601/RFC3339 format, leaving everything else implementation specific. It just happens that all major implementations support the formats I showed.

Contributor guide

Tech stack
javascript
Domain
tooling
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
understanding of JavaScript Date objectfamiliarity with ES6 spec §20.3.1.16experience with polyfill development
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Research the ES6 specification for date time string format (section 20.3.1.16) and compare with ES5. The polyfill must modify the behavior of `new Date(string)` and `Date.parse(string)` to interpret missing timezone as local time instead of UTC. Check the core js source code for existing date polyfills (e.g., `es/date/index.js`) and understand the polyfill architecture. Review the linked Babel issue (babel/babel#1418) for additional context. The solution should only alter parsing for strings without timezone offset, maintaining other cases unchanged.