It's always useful to provide some sample data for questions like this. I'll make up some data for now.
Hopefully this example will help will show you what to do to get what you want with your actual data.
a = datetime('now') + duration(0, [0:120], 0, 0); % Make up some test data
b = linspace(0, 10, numel(a)) + rand(1, numel(a));
hl = plot(a,b);
grid on
xlabel 'Time'
ylabel 'Temperature'
% pick a time point an hour from now for testing - replace with your
% desired time
desired_time = datetime('now') + hours(1);
temp_at_desired_time = b(find(a>desired_time, 1));
datatip(hl, desired_time, temp_at_desired_time);
