Why input string format matters for JavaScript (and moment.js) dates

Original gist for this post

Given this format, '2017-05-01', I want the date of May 1, 2017.

Try it:

new Date('2017-05-01')

Turns out that Date() processes that format as UTC based, not local based. And that returns Sun Apr 30 2017 17:00:00 GMT-0700 – 7 hours behind – which is not what we want.

Try it without the leading zero on the month,

new Date('2017-5-01')

and that returns Mon May 01 2017 00:00:00 GMT-0700 which is what we want.

Try it with stroke/slash characters in place of the hyphens:

new Date('2017/05/01')

That works, too: Mon May 01 2017 00:00:00 GMT-0700

Recommended format: YYYY/MM/DD

Recommended format for parsing is YYYY/MM/DD – user local settings independent. It always assumes local timezone offset for date string given as a parameter.

More on this at “JavaScript Date: a Bad Part” by Jakub Jedryszek.

JavaScript Date vs. Moment.js

This came up today as I was debugging some Moment.js date/time library warnings. The recommended ‘YYYY/MM/DD’ format of the date string param, which gives us what we want from the JavaScript Date() constructor, produces something unexpected in Moment.


Result is a long console warning.

Deprecation warning: value provided is not in a recognized RFC2822 or ISO format. moment construction falls back to js Date(), which is not reliable across all browsers and versions. Non RFC2822/ISO date formats are discouraged and will be removed in an upcoming major release. Please refer to http://momentjs.com/guides/#/warnings/js-date/ for more info.

Turn that off by using Forgiving Mode, i.e., by adding a second parameter, indicating the format, possibly “non-standard”, to be expected by Moment.

Moment('2017/05/01', 'YYYY/MM/DD')

Midnight-minus-one-hour Conclusions

  • Specifications (RFC2822/ISO) and implementations (Date(), Moment.js) are two different things.
  • Time formatting is really hard to codify once and be done with.
  • It can take a lot of time to read up and understand the issues completely.
  • (Author’s message: Use test-driven development to catch this stuff. I first detected these issues today using TDD (For the record, I’m transforming a JSON response into a proper structure to be consumed by d3.js for some line/series charts. TDD, which had seemed a bit costly here, actually cut short the guesswork today).

One thought on “Why input string format matters for JavaScript (and moment.js) dates

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