Convert spectrogram time to UTC

starttime = datetime('18:33:01 UTC 6th July 2013', 'InputFormat', 'HH:mm:ss ''UTC'' dd''th'' MMMM yyyy', 'TimeZone', 'UTC');

event_times = starttime + seconds(t);
plot(event_times, ...)

One hopes that in reality you have your starting time in a format that is easier to parse: the above is a bit weak for the possibility of "1st" or "2nd" or "21st" or "22nd" or "23rd".