@kcastellino/relative-time-element v0.0.2

Extensions for the built-in <time> element

This module is a slight respin on the excellent @github/relative-time-element. If you're looking for something which is battle-tested and production-ready, you should use their module. If you want to find out more about how this module differs from that version, head to the section below.

Formats a timestamp as a localized string or as relative text that auto-updates in the user's browser.

This allows the server to cache HTML fragments containing dates and lets the browser choose how to localize the displayed time according to the user's preferences. For example, the server may have cached the following generated markup:

<time is="t-local" datetime="2014-04-01T16:30:00-08:00">
  April 1, 2014 4:30pm
</time>

Every visitor is served the same markup from the server's cache. When it reaches the browser, the custom time is="t-local" element JavaScript localizes the element's text into the local timezone and formatting.

<time is="t-local" datetime="2014-04-01T16:30:00-08:00">
  1 Apr 2014 21:30
</time>

Dates are displayed before months, and a 24-hour clock is used, according to the user's browser settings.

If the browser's JavaScript is disabled, the default text served in the cached markup is still displayed.

So how does this differ from GitHub's version?

Essentially, it extends the default <time> element using a customized built-in element to attach the JavaScript behavior, rather than using an autonomous custom element (which use tags like <relative-time>).

The primary advantage of customizing the built-in time element is because that element includes added semantic meaning defined by the HTML specification. Using the time element allows automated programs to access a machine-readable version of the date/time value (using the datetime attribute), which is great for things like listing the publish date of a blog post, informing when an event is going to take place, and highlighting other key dates as a date.

The primary disadvantage of customizing the built-in time element is that the Safari web browser (which includes all browsers on iOS) has refused to support this extension mechanism. If you're looking for a solution which works everywhere out-of-the-box, I would recommend @github/relative-time-element, on which this module is based. For more info, go to the Safari browser compatibility section.

Installation

Available on npm as @kcastellino/relative-time-element.

npm install @kcastellino/relative-time-element

This element produces a customized built-in element based on the <time> element, which is not available on Safari. You will need to introduce the appropriate polyfill for customized built-in elements.

This element also uses the Intl.DateTimeFormat & Intl.RelativeTimeFormat APIs, which are supported by all modern JS engines. If you need to support an older browser, you may need to introduce a polyfill for Intl.DateTimeFormat & Intl.RelativeTimeFormat.

Details on these polyfills can be found at the bottom of this README.

Usage

Add a <time is="t-relative"> element to your markup. Provide a default formatted date as the element's text content (e.g. April 1, 2014). It also MUST have a datetime attribute set to an ISO 8601 formatted timestamp.

<time is="t-relative" datetime="2014-04-01T16:30:00-08:00">
  April 1, 2014
</time>

Depending on how far in the future this is being viewed, the element's text will be replaced with one of the following formats:

• 6 years from now
• 20 days from now
• 4 hours from now
• 7 minutes from now
• just now
• 30 seconds ago
• a minute ago
• 30 minutes ago
• an hour ago
• 20 hours ago
• a day ago
• 20 days ago
• on Apr 1, 2014

So, a relative date phrase is used for up to a month and then the actual date is shown.

Attributes

*: If unspecified, formatStyle will return 'narrow' if format is 'elapsed' or 'micro', 'short' if the format is 'relative' or 'datetime', otherwise it will be 'long'.

**: If unspecified, month will return the same value as formatStyle whenever format is 'datetime', otherwise it will be 'short'.

***: If unspecified, weekday will return the same value as formatStyle whenever format is 'datetime', otherwise it will be undefined.

****: If unspecified, year will return 'numeric' if datetime represents the same year as the current year. It will return undefined if unspecified and if datetime represents a different year to the current year.

datetime (string)

This is the datetime that the element is meant to represent. This must be a valid ISO8601 DateTime. It is also possible to use the date property on the element to set the date. el.date expects a Date object, while el.datetime expects a string. Setting one will override the other.

<time is="t-relative" datetime="2014-04-01T16:30:00-08:00" tense="past">
  April 1, 2038 <!-- Will display "now" until April 1 2038 at 16:30:01! -->
</time>
<script>
  const el = document.querySelector('time[is=t-relative]')
  console.assert(el.date.toISOString() === el.datetime)
  el.date = new Date()
  console.assert(el.datetime !== "2014-04-01T16:30:00-08:00")
</script>

format ('datetime'|'relative'|'duration'|'auto'|'micro'|'elapsed')

Format can be either 'datetime', 'relative', or 'duration'. It can also be one of several deprecated formats of 'auto', 'micro', or 'elapsed'.

The default format is auto, which is an alias for relative. In the next major version this will be relative.

format=datetime

The datetime format will display a localised datetime, based on the other properties of the element. It uses Intl.DateTimeFormat to display the datetime in a localised format.

Unless specified, it will consider weekday to be 'long', month to be 'long', and 'year' to be numeric if the datetime is the same as the given year. Overriding formatStyle will change both weekday and month default values. Examples of this format with the default options and an en locale:

• Wed, 26 Aug 2021
• Sat, 31 Dec (assuming the datetime is same year as the current year)

format=relative

The default relative format will display dates relative to the current time (unless they are past the threshold value - see below). The values are rounded to display a single unit, for example if the time between the given datetime and the current wall clock time exceeds a day, then the format will only output in days, and will not display hours, minutes or seconds. Some examples of this format with the default options and an en locale:

• in 20 days
• 20 days ago
• in 1 minute
• on 31 Aug (assuming the current date is the same year as the current year, and is more than 30 days away from 31 Aug)
• on 26 Aug 2021 (assuming the current date is more than 30 days away from 26 Aug 2021)

format=duration

The duration format will display the time remaining (or elapsed time) from the given datetime, counting down the number of years, months, weeks, days, hours, minutes, and seconds. Any value that is 0 will be omitted from the display by default. Examples of this format with the default options and an en locale:

• 4 hours, 2 minutes, 30 seconds
• 4 hours
• 8 days, 30 minutes, 1 second

Deprecated Formats

format=elapsed

This is similar to the format=duration, except the formatStyle defaults to narrow. Code that uses format=elapsed should migrate to format=duration formatStyle=narrow, as it will be removed in a later version.

format=auto

This is identical to format=relative. Code that uses format=auto should migrate to format=relative as this will be the new default in a later version.

format=micro

The micro format which will display relative dates (within the threshold) in a more compact format. Similar to relative, the micro format rounds values to the nearest largest value. Additionally, micro format will not round lower than 1 minute, as such a datetime which is less than a minute from the current wall clock time will display '1m'.

Code that uses format=micro should consider migrating to format=relative (perhaps with formatStyle=narrow), as format=micro can be difficult for users to understand, and can cause issues with assistive technologies. For example some screen readers (such as VoiceOver for mac) will read out 1m as 1 meter.

Cheatsheet

tense ('auto'|'past'|'future', default: auto)

If format is 'datetime' then this value will be ignored.

Tense can be used to prevent duration or relative formatted dates displaying dates in a tense other than the one specified. Setting tense=past will always display future relative dates as now and duration dates as 0 seconds, while setting it to future will always display past dates relative as now and past duration dates as 0 seconds.

For example when the given datetime is 40 seconds behind of the current date:

<time is="t-relative" datetime="2038-04-01T16:30:00-08:00" tense="past">
  April 1, 2038 <!-- Will display "now" until April 1 2038 at 16:30:01! -->
</time>

<time is="t-relative" datetime="1970-04-01T16:30:00-08:00" tense="future">
  April 1, 2038 <!-- Will display "now" unless you had a time machine and went back to 1970 -->
</time>

precision ('year'|'month'|'day'|'hour'|'minute'|'second', default: 'second')

If format is datetime then this value will be ignored.

Precision can be used to limit the display of an relative or duration formatted time. By default times will display down to the second level of precision. Changing this value will truncate the display by zeroing out any unit lower than the given unit, as such units smaller than the given unit won't be displayed during duration, and relative will display now if the time away from the current time is less than the given precision unit.

threshold (string, default: P30D)

If tense is anything other than 'auto', or format is 'relative' (or the deprecated 'auto' or 'micro' values), then this value will be ignored.

Threshold can be used to specify when a relative display (e.g. "5 days ago") should turn into an absolute display (i.e. the full date). This should be a valid ISO8601 Time Duration. If the difference between the current time and the specified datetime is more than the duration, then the date will be displayed as an absolute value (i.e. the full date), otherwise it will be formatted to a relative display (e.g. "5 days ago").

The default value for this is P30D, meaning if the current time is more than 30 days away from the specified date time, then an absolute date will be displayed.

<time is="t-relative" datetime="1970-04-01T16:30:00-08:00" threshold="P100Y">
  <!-- Will display "<N> years ago" until 2070 when it will display "on April 1, 1970" -->
</time>

<time is="t-relative" datetime="1970-04-01T16:30:00-08:00" threshold="P0S">
  <!-- Will always display "on April 1, 1970" -->
</\time>

prefix (string, default: 'on')

If tense is anything other than 'auto', or format is anything other than 'relative' (or the deprecated 'auto' or 'micro' values), then this value will be ignored.

When formatting an absolute date (see above threshold for more details) it can be useful to prefix the date with some text. The default value for this is on but it can be any string value, an will be prepended to the date.

<time is="t-relative" datetime="1970-04-01T16:30:00-08:00" prefix="this happened on">
  <!-- Will always display "this happened on April 1, 1970" -->
</time>

formatStyle ('long'|'short'|'narrow', default: 'narrow'|'long')

This will used to determine the length of the unit names. This value is passed to the Intl objects as the style option. Some examples of how this can be used:

second, minute, hour, weekday, day, month, year, timeZoneName

For dates outside of the specified threshold, the formatting of the date can be configured using these attributes. The values for these attributes are passed to Intl.DateTimeFormat:

lang

Lang is a built-in global attribute. Relative Time will use this to provide an applicable language to the Intl APIs. If the individual element does not have a lang attribute then it will traverse upwards in the tree to find the closest element that does, or default the lang to en.

Browser Support

Browsers without native custom element support, including support for customized built-in elements, require a polyfill. This includes all versions of Safari, since that browser does not support customized built-in elements, as well as all browsers on all versions of iOS, since WebKit is the only browser engine permitted on that platform.

Browsers without native support for Intl.RelativeTimeFormat or Intl.DateTimeFormat (such as Safari 13 or Edge 18) will also need polyfills.

• Chrome
• Firefox
• Microsoft Edge (version 79 and above)

Safari and iOS

Safari does not support customized built-in elements (elements like <time is="t-local">), and the way things currently stand, that's not going to change (see WebKit bug #28544 and their current standards position). Without any fallbacks, the custom element will not be registered, and the default text in the HTML will be displayed. If you require a displayed time to always be localised and cannot rely on a polyfill, you should not use this element in production.

The @ungap/custom-elements polyfill can be used to provide support for customized built-in elements in browsers as old as Safari 8 and Internet Explorer 11. Alternatively, you can use @github/relative-time-element, which uses an autonomous custom element supported in all modern browsers. If you want the semantic benefits of using the time element, you can easily wrap the custom element provided by that module:

<time datetime="2014-04-01T16:30:00-08:00">
  <relative-time datetime="2014-04-01T16:30:00-08:00">
    April 1, 2014 4:30PM PDT
  </relative-time>
</time>

Remember to include the datetime attribute on both elements, with the same value.

See Also

Most of this implementation is based on Basecamp's local_time component. Thanks to @javan for open sourcing that work and allowing for others to build on top of it.

@rmm5t's jquery-timeago is one of the old time-ago-in-words JS plugins.