Line 253 "start at a particular instant of time UTC" does not read well. Where is a definition of the acronym UTC? Starting with what is UTC (a single sentence) would make the rest of this section more clear. Is there a normative ref or informative ref?

Section re-written Coordinated Universal Time (abbreviated UTC) is a time standard based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation.Time zones around the world can be expressed as positive or negative offsets from UTC. When 2 or more parties attempt to agree on a time, e.g., for a meeting, or when to provide a service, they agree to start at a particular instant of time UTC. They agree on that instant in time by converting from local time, e.g., they want a meeting to start at 13:00 Eastern, 18:00 UK. Our lives and the use of services are bound by local time not by UTC. Experientially, local time is the invariant and UTC is mapped on to it. If a government modifies the rules we adjust the mappings and we shift the UTC time. We still want to meet at 13:00 local or have the heating start at 07:00. As long as the rules never change this causes no confusion—but they do. Recent experience has included considerable efforts when the rules for the start of Daylight Saving Time (DST) have changed. If all information is in UTC, and no record of the events basis in the local time and time zone remains, there is no way to re-compute existing contracts. It is often necessary to know if UTC was calculated based on an old or a new rule. A triplet of Local time + timezoneid + (UTC or offset) always allows the determination if a time is valid. If a recalculation of UTC for that local time + tzid results in a different value from that stored then presumably the DST rules have changed since the data was stored. If one can detect that the scheduled time is no longer valid, one can take corrective action. The Technical Committee makes no representation as whether UTC or local time are more appropriate for a given interaction. Because WS-Calendar is based on [iCalendar] , business practices built upon WS-Calendar can support either. For a fuller discussion of time zones, consult [Time Service Recommendations] and [Time Zone Service] in the non-normative references.