CODEWITHSUNDEEP
javascriptv8
Robert Rossmann
Robert Rossmann

Reputation: 12129

Date.parse(0) returns midnight of 2000, why?

When I try to Date.parse() an integer or string 0, it returns 946681200000, which translates to a date of:

Sat Jan 01 2000 00:00:00 GMT+0100 (CET)

Why?

I would assume that the parser interprets the single zero as a year 2000, but the specs say nothing about single-character year definition - both RFC 2822 and ISO 8601 require a four-character year in the string.

I would like to better understand how the string '0' is parsed into a Date, why is it accepted as a valid Date (should it not be NaN or some such?) and why the year 2000 is chosen instead of for example 1900.

Update

After some trial & error, I discovered that the single number is in fact interpreted differently in different numeric ranges.

Upvotes: 6

Views: 771

Answers (2)

Tejas Kale
Tejas Kale

Reputation: 415

As Bergi correctly points out, the spec leaves it up to the implementation to return a date when it is not one of the standard formats.

Here's how its implemented in Chromium's V8 engine:

DateParser.cc

if (!is_iso_date_) {
if (Between(year, 0, 49)) year += 2000;
else if (Between(year, 50, 99)) year += 1900;
}

On Chrome 41.0.2272.76 :

Date.parse(0) returns 946665000000 which is Sat Jan 01 2000 00:00:00

Date.parse(49) returns 2493052200000 which is Fri Jan 01 2049 00:00:00

Date.parse(50) returns -631171800000 which is Sun Jan 01 1950 00:00:00

(The translated times are GMT+5.30; The millisecond value will change depending upon your timezone)

Firefox returns NaN for all of these cases.

Upvotes: 0

Bergi
Bergi

Reputation: 665122

the specs say nothing about single-character year definition

The spec says:

If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats.

For V8 specifically, see this bug report on unpredictable results when called with a single number. You can also read the source directly (dateparser.cc, dateparser.h, dateparser-inl.h).

Upvotes: 8

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