Thunderbird's calendar allows for entering events with a specification of the time as referring to a different time zone than the default. However, the feature opens a map and a listin which the time zones are to be selected by names. While this may be straightforward in most cases, in some others, it may be difficult. For en even I have to schedule, I do not know the time zone. Looking up the place in Wikipedia yielded "UTC+x", not a name fpr the timezone. So I suggest (and request) to add UTC-relations for specifying times.