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Jun 16, 2025
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Concept dates: improve introduction #2680

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41 changes: 20 additions & 21 deletions concepts/dates/about.md
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Original file line number Diff line number Diff line change
Expand Up @@ -50,36 +50,35 @@ The **only** format that is consistent across implementations is the [simplified
A moment in time at [UTC][defn-gmt] looks like this:

```text
YYYY-MM-DDTHH:MM:SSZ
YYYYMMDDTHHMMSSZ
YYYY-MM-DDTHH:mm:ss.mssZ
YYYYMMDDTHHmmss.mssZ
```

Where the following substitutions take place:

| Key | Description |
| ---- | ------------------------------------------- |
| YYYY | The calendar year, represented in 4 digits |
| MM | The calendar month, represented in 2 digits |
| DD | The calendar day, represented in 2 digits |
| HH | The hours in a 24-hour clock, 2 digits |
| MM | The minutes, 2 digits |
| SS | The seconds, 2 digits |
| Key | Description | Default |
| ---- | --------------------------------------------- | ------- |
| YYYY | The calendar year, represented in 4 digits | |
| MM | The calendar month, represented in 2 digits | 01 |
| DD | The calendar day, represented in 2 digits | 01 |
| T | A literal letter T, separating date from time | |
| HH | The hours in a 24-hour clock, 2 digits | 00 |
| mm | The minutes, 2 digits | 00 |
| ss | The seconds, 2 digits | 00 |
| mss | The milliseconds, 3 digits | 000 |
| Z | A literal letter Z, or an offset `+/-HH:mm` | |

The letter `T` separates the date from the time.
The letter `Z` indicates UTC (no timezone, no Day Light Savings).
The literal letter `Z` indicates UTC (no timezone, no Day Light Savings).

<!-- prettier-ignore -->
~~~exercism/caution
Other formats that are accepted by `Date.parse` may or may not work.
When working with Dates in JavaScript, _always_ use an ISO 8601 timestamp when converting from a `string` to a `Date`.
Because there are default values for most components, leaving parts off at the end is valid:

Date-only forms are allowed, but not all ISO 8601 formats are supported.
Consult the [simplified version explanation page on MDN][mdn-date-string-format].
```text
YYYY-MM-DD
```

[mdn-date-string-format]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date#date_time_string_format
~~~
Defaults to a time of 00:00:00.000

If the timestamp does not end in `Z`, and it does not end with `+HH:MM` or `-HH:MM`, indicating a timezone offset, because of historical reasons, the following applies:
If the timestamp does not end in `Z`, and it does not end with `+HH:mm` or `-HH:mm` (indicating a timezone offset), because of historical reasons, the following applies:

> When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.
> The interpretation as a UTC time is due to a historical spec error that was not consistent with ISO 8601 but could not be changed due to web compatibility.
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41 changes: 20 additions & 21 deletions concepts/dates/introduction.md
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Original file line number Diff line number Diff line change
Expand Up @@ -50,36 +50,35 @@ The **only** format that is consistent across implementations is the [simplified
A moment in time at [UTC][defn-gmt] looks like this:

```text
YYYY-MM-DDTHH:MM:SSZ
YYYYMMDDTHHMMSSZ
YYYY-MM-DDTHH:mm:ss.mssZ
YYYYMMDDTHHmmss.mssZ
```

Where the following substitutions take place:

| Key | Description |
| ---- | ------------------------------------------- |
| YYYY | The calendar year, represented in 4 digits |
| MM | The calendar month, represented in 2 digits |
| DD | The calendar day, represented in 2 digits |
| HH | The hours in a 24-hour clock, 2 digits |
| MM | The minutes, 2 digits |
| SS | The seconds, 2 digits |
| Key | Description | Default |
| ---- | ------------------------------------------- | ------- |
| YYYY | The calendar year, represented in 4 digits | |
| MM | The calendar month, represented in 2 digits | 01 |
| DD | The calendar day, represented in 2 digits | 01 |
| T | A literal letter T, separating date & time | T |
| HH | The hours in a 24-hour clock, 2 digits | 00 |
| mm | The minutes, 2 digits | 00 |
| ss | The seconds, 2 digits | 00 |
| mss | The milliseconds, 3 digits | 000 |
| Z | A literal letter Z, or an offset `+/-HH:mm` | |

The letter `T` separates the date from the time.
The letter `Z` indicates UTC (no timezone, no Day Light Savings).
The literal letter `Z` indicates UTC (no timezone, no Day Light Savings).

<!-- prettier-ignore -->
~~~exercism/caution
Other formats that are accepted by `Date.parse` may or may not work.
When working with Dates in JavaScript, _always_ use an ISO 8601 timestamp when converting from a `string` to a `Date`.
Because there are default values for most components, leaving parts off at the end is valid:

Date-only forms are allowed, but not all ISO 8601 formats are supported.
Consult the [simplified version explanation page on MDN][mdn-date-string-format].
```text
YYYY-MM-DD
```

[mdn-date-string-format]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date#date_time_string_format
~~~
Defaults to a time of 00:00:00.000

If the timestamp does not end in `Z`, and it does not end with `+HH:MM` or `-HH:MM`, indicating a timezone offset, because of historical reasons, the following applies:
If the timestamp does not end in `Z`, and it does not end with `+HH:mm` or `-HH:mm` (indicating a timezone offset), because of historical reasons, the following applies:

> When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.
> The interpretation as a UTC time is due to a historical spec error that was not consistent with ISO 8601 but could not be changed due to web compatibility.
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46 changes: 28 additions & 18 deletions exercises/concept/appointment-time/.docs/introduction.md
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Original file line number Diff line number Diff line change
Expand Up @@ -50,23 +50,39 @@ The **only** format that is consistent across implementations is the [simplified
A moment in time at [UTC][defn-gmt] looks like this:

```text
YYYY-MM-DDTHH:MM:SSZ
YYYYMMDDTHHMMSSZ
YYYY-MM-DDTHH:mm:ss.mssZ
YYYYMMDDTHHmmss.mssZ
```

Where the following substitutions take place:

| Key | Description |
| ---- | ------------------------------------------- |
| YYYY | The calendar year, represented in 4 digits |
| MM | The calendar month, represented in 2 digits |
| DD | The calendar day, represented in 2 digits |
| HH | The hours in a 24-hour clock, 2 digits |
| MM | The minutes, 2 digits |
| SS | The seconds, 2 digits |
| Key | Description | Default |
| ---- | --------------------------------------------- | ------- |
| YYYY | The calendar year, represented in 4 digits | |
| MM | The calendar month, represented in 2 digits | 01 |
| DD | The calendar day, represented in 2 digits | 01 |
| T | A literal letter T, separating date from time | |
| HH | The hours in a 24-hour clock, 2 digits | 00 |
| mm | The minutes, 2 digits | 00 |
| ss | The seconds, 2 digits | 00 |
| mss | The milliseconds, 3 digits | 000 |
| Z | A literal letter Z, or an offset `+/-HH:mm` | |

The letter `T` separates the date from the time.
The letter `Z` indicates UTC (no timezone, no Day Light Savings).
The literal letter `Z` indicates UTC (no timezone, no Day Light Savings).

Because there are default values for most components, leaving parts off at the end is valid:

```text
YYYY-MM-DD
```

Defaults to a time of 00:00:00.000

If the timestamp does not end in `Z`, and it does not end with `+HH:mm` or `-HH:mm` (indicating a timezone offset), because of historical reasons, the following applies:

> When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.
> The interpretation as a UTC time is due to a historical spec error that was not consistent with ISO 8601 but could not be changed due to web compatibility.
> See [Broken Parser – A Web Reality Issue][ref-broken-parser].

<!-- prettier-ignore -->
~~~exercism/caution
Expand All @@ -79,12 +95,6 @@ Consult the [simplified version explanation page on MDN][mdn-date-string-format]
[mdn-date-string-format]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date#date_time_string_format
~~~

If the timestamp does not end in `Z`, and it does not end with `+HH:MM` or `-HH:MM`, indicating a timezone offset, because of historical reasons, the following applies:

> When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.
> The interpretation as a UTC time is due to a historical spec error that was not consistent with ISO 8601 but could not be changed due to web compatibility.
> See [Broken Parser – A Web Reality Issue][ref-broken-parser].

### **Date object**

An existing date object can also be used as a constructor argument.
Expand Down