Can you explain the variances between creating a new date object with input as "2017-01-01" and "2017-1-1" in JavaScript?

Question

Can you explain the variances between creating a new date object with input as "2017-01-01" and "2017-1-1" in JavaScript?

When I use new Date("2017-01-01") in the Chrome console, the output displays its hour as 8. However, when using new Date("2017-01-1") and new Date("2017-1-01"), the hour is shown as 0. This brings up the question of how new Date(dateString) actually parses the input.

new Date("2017-01-01")
// Sun Jan 01 2017 08:00:00 GMT+0800 (China Standard Time)*
new Date("2017-01-1")
// Sun Jan 01 2017 00:00:00 GMT+0800 (China Standard Time)*
new Date("2017-1-1")
// Sun Jan 01 2017 00:00:00 GMT+0800 (China Standard Time)*
new Date("2017-1-01")
// Sun Jan 01 2017 00:00:00 GMT+0800 (China Standard Time)*

javascript

Answer 1

Answer №1

"2017-01-01" adheres to the ES5 Date Time String format (a simplified version of ISO 8601 Extended Format) and is therefore in UTC time, equivalent to 8am in China. In Chrome, all other strings are parsed as local time.

Reference code in Chromium:

Date parsing in Chromium abides by ES5 standards with additional guidelines:

Any unfamiliar word before the first number is disregarded.
Text within parentheses is not considered.
An unsigned number followed by a colon denotes a time value and is included in the TimeComposer. Adding double colons or a period after a number signifies seconds and milliseconds, respectively. Other numbers indicate date components added to DayComposer.
A month name, or any word resembling one, is recorded as a named month in Day composer.
Recognizable time-zone words or patterns like (+|-)(hhmm|hh:) are identified accordingly.
Legacy dates do not allow extra signs or unmatched parentheses following a number, while garbage text preceding the first number is permitted.
Strings conforming to ES5 criteria and the specified rules will be parsed using ES5 regulations. For example, "1970-01-01" will be in the UTC time zone, not local time.

What's the significance?

Note that "2017-01-01" is interpreted in UTC due to being a "date" string without time information, aligning with the ES5 definition for such strings. If specifying a time, it follows the ISO standard and parses in local time.

Examples:

2017-01-01 - Jan 1, 2017 in UTC time
2017-01-01T00:00 - Jan 1, 2017 in local time
2017-1-1 - Jan 1, 2017 in local time
2017-(hello)01-01 - Jan 1, 2017 in local time
may 2017-01-01 - May 1, 2017 in local time
mayoooo 2017-01-01 - May 1, 2017 in local time
```
"jan2017feb-mar01apr-may01jun"
```
- Jun 1, 2017 in local time

Answer 2

"2017-01-01" adheres to the ES5 Date Time String format (a simplified version of ISO 8601 Extended Format) and is therefore in UTC time, equivalent to 8am in China. In Chrome, all other strings are parsed as local time.

Reference code in Chromium:

Date parsing in Chromium abides by ES5 standards with additional guidelines:

Any unfamiliar word before the first number is disregarded.
Text within parentheses is not considered.
An unsigned number followed by a colon denotes a time value and is included in the TimeComposer. Adding double colons or a period after a number signifies seconds and milliseconds, respectively. Other numbers indicate date components added to DayComposer.
A month name, or any word resembling one, is recorded as a named month in Day composer.
Recognizable time-zone words or patterns like (+|-)(hhmm|hh:) are identified accordingly.
Legacy dates do not allow extra signs or unmatched parentheses following a number, while garbage text preceding the first number is permitted.
Strings conforming to ES5 criteria and the specified rules will be parsed using ES5 regulations. For example, "1970-01-01" will be in the UTC time zone, not local time.

What's the significance?

Note that "2017-01-01" is interpreted in UTC due to being a "date" string without time information, aligning with the ES5 definition for such strings. If specifying a time, it follows the ISO standard and parses in local time.

Examples:

2017-01-01 - Jan 1, 2017 in UTC time
2017-01-01T00:00 - Jan 1, 2017 in local time
2017-1-1 - Jan 1, 2017 in local time
2017-(hello)01-01 - Jan 1, 2017 in local time
may 2017-01-01 - May 1, 2017 in local time
mayoooo 2017-01-01 - May 1, 2017 in local time
```
"jan2017feb-mar01apr-may01jun"
```
- Jun 1, 2017 in local time

Answer 3

Answer №2

Exploring the difference between using new Date("2017-01-01") and new Date("2017-1-1")

The syntax new Date("2017-01-01")

strictly adheres to the specifications (details below). On the other hand, when you use new Date("2017-1-1"), it diverges from the standard and relies on any "...implementation-specific heuristics or implementation-specific date formats" that are applied by the JavaScript engine. Consequently, there is uncertainty in how or if the input will be parsed successfully, and if so, whether it will be interpreted as UTC time or local time.

Despite being compliant with the specifications, new Date("2017-01-01") has faced inconsistencies across different browser implementations over time due to its lack of timezone indicator:

In ES5 reference (December 2009), strings lacking a timezone indicator were supposed to be parsed in UTC. This conflicted with the ISO-8601 standard, stating that such strings should be considered local time, not UTC. Hence, in ES5, new Date("2017-01-01") was parsed in UTC.
ES2015 (also known as ES6, June 2015) specified that strings without a timezone indicator should be treated as local time similar to ISO-8601. Consequently, in ES2015, new Date("2017-01-01") was interpreted as local time.
However, this was revised in ES2016 (June 2016) following years of browsers parsing date-only forms with "-" as UTC. Therefore, starting from ES2016, only "date-only" forms like "2017-01-01" are parsed in UTC, while "date/time" forms like "2017-01-01T00:00:00" are processed in local time.

Regrettably, not all JavaScript engines conform precisely to the specifications. For instance, Chrome (version 56 at the time of writing) incorrectly interprets date/time forms as UTC instead of local time (similarly to IE9). Conversely, Chrome, Firefox, and IE11 handle date-only forms correctly by considering them as UTC. Meanwhile, IE8 does not support the ISO-8601 format since it predates the release of the ES5 spec.

Answer 4

Exploring the difference between using new Date("2017-01-01") and new Date("2017-1-1")

The syntax new Date("2017-01-01")

strictly adheres to the specifications (details below). On the other hand, when you use new Date("2017-1-1"), it diverges from the standard and relies on any "...implementation-specific heuristics or implementation-specific date formats" that are applied by the JavaScript engine. Consequently, there is uncertainty in how or if the input will be parsed successfully, and if so, whether it will be interpreted as UTC time or local time.

Despite being compliant with the specifications, new Date("2017-01-01") has faced inconsistencies across different browser implementations over time due to its lack of timezone indicator:

In ES5 reference (December 2009), strings lacking a timezone indicator were supposed to be parsed in UTC. This conflicted with the ISO-8601 standard, stating that such strings should be considered local time, not UTC. Hence, in ES5, new Date("2017-01-01") was parsed in UTC.
ES2015 (also known as ES6, June 2015) specified that strings without a timezone indicator should be treated as local time similar to ISO-8601. Consequently, in ES2015, new Date("2017-01-01") was interpreted as local time.
However, this was revised in ES2016 (June 2016) following years of browsers parsing date-only forms with "-" as UTC. Therefore, starting from ES2016, only "date-only" forms like "2017-01-01" are parsed in UTC, while "date/time" forms like "2017-01-01T00:00:00" are processed in local time.

Regrettably, not all JavaScript engines conform precisely to the specifications. For instance, Chrome (version 56 at the time of writing) incorrectly interprets date/time forms as UTC instead of local time (similarly to IE9). Conversely, Chrome, Firefox, and IE11 handle date-only forms correctly by considering them as UTC. Meanwhile, IE8 does not support the ISO-8601 format since it predates the release of the ES5 spec.

Answer 5

Answer №3

When JavaScript parses dates, it treats ISO dates as UTC time and other date formats as local time.

According to a reference on MDN,

If the string is an ISO 8601 date, arguments are interpreted using the UTC time zone.

For instance, parsing a date string like "March 7, 2014" assumes a local timezone; however, if the format is in ISO like "2014-03-07," it will be assumed to be in UTC timezone (ES5 and ECMAScript 2015). Hence, Date objects created from these strings may correspond to different points in time depending on the version of ECMAScript supported unless the system specifies a local time zone of UTC. This means that two seemingly equivalent date strings could yield two distinct values based on their conversion format.

// When parsed, '2017-03-28' is considered UTC time,
// displayed as '2017-03-28 00:00:00' in UTC timezone,
// displayed as '2017-03-28 06:00:00' in my timezone:
console.log("ISO dates:");
var isoDates = [new Date("2017-03-28")];
for (var dt of isoDates)
{
  console.log(dt.toUTCString() + " / " + dt.toLocaleString());
}

// Other date formats are treated as local time, 
// shown as '2017-03-27 18:00:00' in my timezone,
// shown as '2017-03-28 00:00:00' in my timezone:
console.log("Other formats:");
var otherDates = [new Date("2017-3-28"), new Date("March 28, 2017"), new Date("2017/03/28")];
for (var dt of otherDates)
{
  console.log(dt.toUTCString() + " / " + dt.toLocaleString());
}

Answer 6

When JavaScript parses dates, it treats ISO dates as UTC time and other date formats as local time.

According to a reference on MDN,

If the string is an ISO 8601 date, arguments are interpreted using the UTC time zone.

For instance, parsing a date string like "March 7, 2014" assumes a local timezone; however, if the format is in ISO like "2014-03-07," it will be assumed to be in UTC timezone (ES5 and ECMAScript 2015). Hence, Date objects created from these strings may correspond to different points in time depending on the version of ECMAScript supported unless the system specifies a local time zone of UTC. This means that two seemingly equivalent date strings could yield two distinct values based on their conversion format.

// When parsed, '2017-03-28' is considered UTC time,
// displayed as '2017-03-28 00:00:00' in UTC timezone,
// displayed as '2017-03-28 06:00:00' in my timezone:
console.log("ISO dates:");
var isoDates = [new Date("2017-03-28")];
for (var dt of isoDates)
{
  console.log(dt.toUTCString() + " / " + dt.toLocaleString());
}

// Other date formats are treated as local time, 
// shown as '2017-03-27 18:00:00' in my timezone,
// shown as '2017-03-28 00:00:00' in my timezone:
console.log("Other formats:");
var otherDates = [new Date("2017-3-28"), new Date("March 28, 2017"), new Date("2017/03/28")];
for (var dt of otherDates)
{
  console.log(dt.toUTCString() + " / " + dt.toLocaleString());
}

Answer 7

Answer №4

The date format being used here follows the International Standard (ISO format)

new Date("2017-01-01")

By sticking to this format, you can ensure consistent output across all web browsers.

On the other hand, using different date formats may lead to varying results depending on the browser since they are not as clearly defined.

For example, take a look at how this date format is handled:

new Date("2017-1-1")

This format is accepted in Chrome but triggers an error in IE 11

Answer 8

The date format being used here follows the International Standard (ISO format)

new Date("2017-01-01")

By sticking to this format, you can ensure consistent output across all web browsers.

On the other hand, using different date formats may lead to varying results depending on the browser since they are not as clearly defined.

For example, take a look at how this date format is handled:

new Date("2017-1-1")

This format is accepted in Chrome but triggers an error in IE 11

Can you explain the variances between creating a new date object with input as "2017-01-01" and "2017-1-1" in JavaScript?

Answer №1

What's the significance?

Answer №2

Answer №3

Answer №4

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