Publish-Subscribe (XEP-0060)  and Personal Eventing Protocol (XEP-0163)  can be used to publish a wide variety of "extended presence" information about users. This document specifies an extended presence payload format that communicates information about a user's time zone. For example, this information may be of interest to a user's contacts for displaying notices advising against starting a chat at 4am in the user's local time.
Time zone information can also be obtained through Entity Time (XEP-0202)  by directly querying a full JID for a user. However, given that some clients are chiefly concerned with the time zone offset, and in the interest of reducing floods of IQ requests, we define a way to expose the time zone offset via PEP.
The numeric time zone offset from UTC for the user is expressed as the textual content of the <tzo/> element qualified by the 'urn:xmpp:timezone:0' namespace.  The format MUST conform to the Time Zone Definition (TZD) specified in XMPP Date and Time Profiles (XEP-0082) .
When a user wishes to share time zone information, its client may publish that fact to a PEP node whose NodeID is "urn:xmpp:timezone:0" (see Protocol Namespaces regarding issuance of one or more permanent namespaces). Because time zone information is not pure presence information and can change independently of the user's availability, it SHOULD NOT be provided as an extension to the <presence/> stanza type.
The time zone offset is then delivered to all subscribers:
When the user wishes to stop broadcasting its time zone offset, the user's client SHOULD send an empty <tzo/> element with the same ItemID:
Revealing an entity's numeric time zone offset may leak limited information about the entity's current location. If the entity's understanding of UTC is far off from actual UTC, revealing that discrepancy may make it possible for an attacker to send XML stanzas that appear to be in the past or future even though they are not; therefore an entity should use the Network Time Protocol (RFC 958 ) or a similar technology to stay synchronized with actual UTC.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
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The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
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