Once the figure is plotted, use xlim([x1,x2]) where x1 and x2 are in datetime form to force the graph's x-axis limits be x1 and x2.
See example below:
Where the minimum x-axis value is the thrid entry in the x datetime array and the maximum x-axis value is the last entry
For using datetick, try (if you want to keep the current tick locations):
datetick('x','mm/dd','keeplimits','keepticks')
If not, remove 'keepticks' and use:
set(gca,'XTicks',y) %where y is an array of values you want to use for your xticks in datetime form
xlim([x1,x2])
datetick('x','mm/dd','keeplimits')
