746529 - Date.parse strangeness with millisec/1000 > 0.999

Status: RESOLVED FIXED

(Core :: JavaScript: Standard Library, defect)

Description

[[e-mail]]

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:11.0) Gecko/20100101 Firefox/11.0
Build ID: 20120312181643

Steps to reproduce:

Date.parse("1970-01-01T00:00:00.99999999999999995")
Date.parse("1970-01-01T00:00:00.99999999999999996")

Date.parse("1970-02-01T00:00:00.99999999999999") % 1000
etc etc


Actual results:

(1) returned 999,
(2) returned 1000,
(3) returned 0 (mod 1000).

Note the precision of the millisecond field varies depending on the values of the higher fields (year/month/etc).


Expected results:

In the millisecond field: digits beyond the third should be ignored, i.e. the millisecond field should be rounded down to three decimal places.  Very low order digits should not affect the rounding.

(Then the precision of the millisecond field would also always be 3, regardless of the values of the higher fields.)

Comment 1

[Vinny Diehl]

As far as I can tell, the rounding after the third digit is implementation defined. The spec only makes mention of 3 digits exactly in the Date Time String Format:

https://tc39.es/ecma262/#sec-date-time-string-format

YYYY-MM-DDTHH:mm:ss.sssZ
...
sss is the number of complete milliseconds since the start of the second as three decimal digits.

Clipping after 3 digits rather than rounding is Chrome behavior, and Firefox and Safari round at different points (Safari's behavior is especially bizarre):

Format	JSC	SM	V8
"1970-01-01T00:00:00.9999999999999999990Z"	NaN	1000	999
"1970-01-01T00:00:00.99999999999999996Z"	1000	1000	999
"1970-01-01T00:00:00.99999999999999995Z"	1000	999	999
"1970-01-01T00:00:00.9999999999999999Z"	1000	999	999
"1970-01-01T00:00:00.9999999999999998Z"	999	999	999

Comment 2

[Vinny Diehl]

Attachment: Bug 746529 - Truncate milliseconds after 3 digits r?arai

Previously, we would round up on the 17th digit, this change was made
for Chrome parity.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/81243364a707
Truncate milliseconds after 3 digits r=arai

Comment 4

[Cristian Tuns]

https://hg.mozilla.org/mozilla-central/rev/81243364a707

Comment 5

[Vinny Diehl]

Relnoted here: https://github.com/mdn/content/pull/30234