There are two solutions to your problem
a) choose a format other than datenum for your dates variable, f.ex.
dates = datetime(Time,'InputFormat','yyyy/MM/dd');
b) keep the data the same but fiddle with the xlabel property
myAx = gca; % or via plothandle.Parent
n = 5; % user each n-th element
myAx.XTick = dates(1:n:end);
myAx.XTickLabel = datestr(dates(1:n:end), 'yyyy/MM/dd');