I did not find a way to convert a local juce::Time to UTC. Would it make sense to add something like the member below?
Time Time::toUTC()
{
time_t secondsSinceEpoch = millisSinceEpoch / 1000;
tm utcTm;
gmtime_r(&secondsSinceEpoch, &utcTm);
int64 utcSecondsSinceEpoch = mktime(&utcTm);
return Time(utcSecondsSinceEpoch * 1000);
}