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

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

https://github.com/angular/angular/blob/d1ea1f4c7f3358b730b0d94e65b00bc28cae279c/packages/common/src/i18n/format_date.ts#L735

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 is no limit on the number of decimal places for the decimal fraction https://momentjs.com/docs/#:~:text=Fractional Second https://date-fns.org/v2.14.0/docs/format#:~:text=Fraction of second

🔬 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]

So I think it would be reasonable to “fix” this, but I think it would count as a breaking change and not be released until 11.0.0

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