MSC4175: Profile field for user time zone

proposals/4175-profile-field-time-zone.md

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+# MSC4175: Profile field for user time zone
+
+Knowing another user's time zone is useful for knowing whether they are likely
+to respond or not. Example uses include:
+
+* Showing a user's time zone or time zone offset directly.
+* Showing a user's local time (with hints of whether it is day or night there).
+
+
+## Proposal
+
+Profiles can provide an optional `m.tz` field with values equal to names from the
+[IANA Time Zone Database](https://www.iana.org/time-zones).
+Clients can set and fetch this via the [normal API endpoints](https://spec.matrix.org/v1.14/client-server-api/#profiles).
+
+* Servers MAY validate that the value is a valid IANA time zone. If deemed invalid
+  they MUST return a 400 error with error code `M_INVALID_PARAM`.
+* Clients MUST handle invalid or unknown values. One approach may be processing the value as though it was never set.
+
+The rationale for somewhat loose validation is that different clients/servers may have
+different understanding of valid time zones, e.g. different versions of the time zone
+database.
+
+If the field is not provided it SHOULD be interpreted as having no time zone information
+for that user.
+
+
+## Potential issues
+
+Clients may need to understand IANA time zone names to show useful information to users.
+Some languages make this easy, e.g. JavaScript can handle this using
+[`Date.toLocaleString()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toLocaleString).
+This may cause clients to bundle the IANA time zone database (and thus also keep it
+up to date).
+
+Using the IANA time zone name has the downside that it does now allow arbitrary offsets,
+which may be required for time zones which are not internationally recognized.
+
+Clients will need to manually update the profile field when the user changes time zone.
+This could be automated by clients based on location, or left as a manual change to
+users.
+
+Clients may wish to periodically fetch the time one of other users as it is
+liable to change somewhat frequently. Currently, profile data isn't propagated/synchronized
+between servers, but that's left to a future MSC to solve. It is recommended that
+clients cache the value for 12 - 24 hours.
+
+There should not be backwards compatibility concerns since clients should be ignoring
+unknown profile fields.
+
+
+## Alternatives
+
+The time zone offset could be included directly (in minutes/seconds or in `[+-]HH:MM` form).
+This would require clients to manually update the profile field during daylight
+savings. Using the IANA time zone name is robust against this.
+
+
+### Delegate profile fields
+
+There are several standards related to storing of contact information electronically,
+notably vCard and its derivatives (see below). It is unclear if Matrix profile
+information is similar enough to the contact information found in vCard to warrant using
+that format directly, although there is certainly some overlap.
+
+Some of the JSON formats for vCard which include time zone information are detailed below:
+
+[RFC7095: jCard The JSON Format for vCard](https://datatracker.ietf.org/doc/html/rfc7095)
+format could be used instead, but this doesn't make much sense unless the entire
+profile was replaced.
+
+[RFC9553](https://datatracker.ietf.org/doc/html/rfc9553) offers an alternative
+representation for contacts (which is not backwards compatible with vCard). There
+exists `timeZone` field under the `addresses` field which uses an time zone name
+from the IANA Time Zone Database.
+
+Note there's an alternative [jCard](https://microformats.org/wiki/jCard) format
+which is a non-standard derivative of [hCard](https://microformats.org/wiki/hcard).
+
+
+### Competitive analysis
+
+Slack's [`users.info` API call](https://api.slack.com/methods/users.info) includes
+3 separate fields:
+
+* `tz`: the time zone database name (e.g. `"America/New_York"`)
+* `tz_label`: a friendly name (e.g. `"Eastern Daylight Time"`)
+* `tz_offset`: offset in seconds as an integer (e.g. `-14400`)
+
+XMPP uses either:
+
+* [XEP-054](https://xmpp.org/extensions/xep-0054.html) uses vCard
+  ([RFC2426](https://datatracker.ietf.org/doc/html/rfc2426)) converted to XML via
+  [draft-dawson-vcard-xml-dtd-01](https://datatracker.ietf.org/doc/html/draft-dawson-vcard-xml-dtd-01)
+* [XEP-0292](https://xmpp.org/extensions/xep-0292.html) uses xCard: vCard XML Representation
+  ([RFC6351](https://datatracker.ietf.org/doc/html/rfc6351)), see also vCard4
+  ([RFC6351](https://datatracker.ietf.org/doc/html/rfc6351))
+
+Rocket.Chat provides a user's [time zone offset](https://developer.rocket.chat/docs/user)
+in the `utcOffset` field.
+
+Mattermost [returns an object](https://api.mattermost.com/#tag/users/operation/GetUser)
+with the user's manual and/or automatic IANA time zone name.
+
+Discord, Twitter, and IRC don't provide a user's time zone.
+
+
+## Security considerations
+
+Showing a user's time zone gives some information to their location. There is currently
+no way to limit what profile fields other users can see.
+
+Clients may wish to warn users when providing a timezone and give
+the option to not include it in their profile.
+
+
+## Unstable prefix
+
+`us.cloke.msc4175.tz` should be used in place of `m.tz`. 
+
+Clients may immediately use the stable profile field once this MSC is accepted. This is
+a client-to-client protocol and no feature negotiation is necessary.
+
+
+## Dependencies
+
+Requires [MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133).