bug(common): date pipe rounds up date when fractional second is more than 3 digits

[antoinebrault]

Hi,

I believe I've come across a bug in the date parsing.
I haven't seen any indications that this is expected behavior, so I'm opening an issue here.

🐞 bug report

Affected Package

@angular/common
Seems to be in every version I tested. Angular 6, 9, 10

angular/packages/common/src/i18n/format_date.ts

Line 735 in d1ea1f4

const ms = Math.round(parseFloat('0.' + (match[7] || 0)) * 1000);

Is this a regression?

Not that I've seen.

Description

The Date pipe has some unexpected behavior when second fractions have more than 3 digits.

ISO 8601 format is not capped on fractional second number of digits.
https://www.w3.org/TR/NOTE-datetime
https://en.wikipedia.org/wiki/ISO_8601#:~:text=There%20is%20no%20limit%20on%20the%20number%20of%20decimal%20places%20for%20the%20decimal%20fraction
https://momentjs.com/docs/#:~:text=Fractional%20Second
https://date-fns.org/v2.14.0/docs/format#:~:text=Fraction%20of%20second

🔬 Minimal Reproduction

https://stackblitz.com/edit/angular-date-fractional-second-test

  3 digits: {{ '2020-08-01T23:59:59.999' | date: 'yyyy-MM-dd' }} 
  <br>
  4 digits: {{ '2020-08-01T23:59:59.9999' | date: 'yyyy-MM-dd' }}

[petebacondarwin]

I am not sure of the history behind this but I think it might be by design. I note that the ISO 8601 specification says:

There is no limit on the number of decimal places for the decimal fraction. However, the number of decimal places needs to be agreed to by the communicating parties. For example, in Microsoft SQL Server, the precision of a decimal fraction is 3, i.e., "yyyy-mm-ddThh:mm:ss[.mmm]".

In other words the number of decimal places is implementation specific. And it appears that Angular has made the decision that it only accepts 3 decimal places.

In the Angular docs there is a reference to the W3C Date and Time document which restricts the ISO 8601 standard to a smaller number of formats. In particular it says:

This profile does not specify how many digits may be used to represent the decimal fraction of a second. An adopting standard that permits fractions of a second must specify both the minimum number of digits (a number greater than or equal to one) and the maximum number of digits (the maximum may be stated to be "unlimited").

That suggests that we should specify in our documentation what the maximum number of digits supported is. It is arguable that the table of "custom" formats in the DatePipe page implies that we only support 1, 2 and 3 digits, but I think we could/should be more clear about how we parse strings passed to DatePipe in order to then be rendered.

So I would say that this is not actually a bug in Angular, but instead either a documentation bug or a feature request to support more fraction digits.