I would use datetime and string.
mydate = 20190606; % Current yearmonthday (8 digits)
myrun = 06; % Current hour 00 06 12 18Z (2 digits)
myforecasthour = 033:043;
Convert your mydate data into a datetime by telling datetime that the input is a number in the yyyymmdd format. Make sure to specify that you're using UTC. Specify the Format you want the datetime to use to display itself.
startDate = datetime(mydate, 'ConvertFrom', 'yyyymmdd', ...
'TimeZone', 'UTC', ...
'Format', 'MMM d yyyy HX')
Add in the hours given as myrun and the hours given for the start and end of the myforecasthour vector to give the start and end times of the forecast.
forecastrange = startDate + hours(myrun) + hours(myforecasthour([1 end]))
Convert each datetime to a string and combine it with "to" then use the resulting string as the title of your axes.
title(string(forecastrange(1)) + " to " + string(forecastrange(2)));
