The UTC time is defined in the specification CAT v2.0.2 by the following rule
<UTC-time> ::= <time-hour> ":" <time-minute> ":" <time-second> "Z"
Thus only Zero(or Zulu) zone is allowed.
The following change was accepted by the OGC Catalog Work Group. It includes the "time-zone-offset" which is optional.
<time-zone-offset> ::= "Z" | <sign> <time-hour>":"<time-minute>
Thus, Zero zone is allowed like in the precedent rule, but a time zone can be include like +03:00. If <time-zone-offset> is not given then local time is assumed.