Why isn't "2016-02-16" equal to "2016-02-16 00:00"?

I'm trying to pass both date strings to new Date(t).

I expect both strings represent the same time, after all, if I omit the time, shouldn't it be midnight of that day?

But while,

new Date("2016-02-16 00:00")

returns 2016-02-16, midnight, local time as expected,

new Date("2016-02-16")

returns 2016-02-16, midnight UTC, which is wrong, or at least not what I expected given what the other string parses as.

I would understand it if they would both have the same behavior, whether it is to return the time as local time, or as UTC, but it seems very inconsistent why they return different things like this.

As a workaround, whenever I encounter a date that has no corresponding timestamp I can append " 00:00" to get consistent behavior, but it seems like this is rather fragile.

I am getting this value from an INPUT element, of type 'datetime-local' so it seems especially inconsistent that I have to work around a value returned by a page element.

Am I doing something wrong, or should I be doing something differently?

Solution 1:

It's what the ES5.1 specification says to do:

The value of an absent time zone offset is “Z”.

It also says:

The function first attempts to parse the format of the String according to the rules called out in Date Time String Format (15.9.1.15). If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats.

Since the format requires a T separator between date and time, the valid times go to UTC:

> new Date("2016-02-16T00:00:00")
Tue Feb 16 2016 01:00:00 GMT+0100 (CET)
> new Date("2016-02-16")
Tue Feb 16 2016 01:00:00 GMT+0100 (CET)

...while in node.js, an invalid time (without the T separator) seems to go to the implementation specific localtime:

> new Date("2016-02-16 00:00:00")
Tue Feb 16 2016 00:00:00 GMT+0100 (CET)

Note that ES6 changed this, in the same part of the documentation it changes to:

If the time zone offset is absent, the date-time is interpreted as a local time.

The joy of breaking changes.

Edit

According to TC39, the specification is meant to be interpreted as date and time strings without a time zone (e.g. "2016-02-16T00:00:00") are treated as local (per ISO 8601), but date only strings (e.g. "2016-02-16") as UTC (which is inconsistent with ISO 8601).

Solution 2:

According to the specifications:

The function first attempts to parse the format of the String according to the rules called out in Date Time String Format (15.9.1.15). If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats.

And Date Time String Formats accept 2016-02-16 as a valid date

This format includes date-only forms:

YYYY
YYYY-MM
YYYY-MM-DD

[...] If the HH, mm, or ss fields are absent “00” is used as the value and the value of an absent sss field is “000”. The value of an absent time zone offset is “Z”.

Thus 2016-02-16 translates to 2016-02-16T00:00:00.000Z.

The other date 2016-02-16 00:00 does not conform to the format and therefore its parsing is implementation specific. Apparently, such dates are treated as having local time zone and your example date will return different values depending on time zone:

/* tz = +05:00 */ new Date("2016-02-16 00:00").toISOString() // 2016-02-15T19:00:00.000Z
/* tz = -08:00 */ new Date("2016-02-16 00:00").toISOString() // 2016-02-16T08:00:00.000Z

Summary:

• For conforming date time formats the behavior is well defined — in the absence of time zone offset the date string is treated as UTC (ES5) or local (ES6).
• For non-conforming date time formats the behavior is implementation specific — in the absence of time zone offset the usual behavior is to treat the date as local.
• As a matter of fact, the implementation could choose to return NaN instead of trying to parse non-conforming dates. Just test your code in Internet Explorer 11 ;)