You want to tell the axes to place the ticks wherever it thinks is best via the XTickMode property. You also want to tell the axes to decide automatically how to label those ticks using the XTickLabelMode property.
Here's the statement to do it:
set(gca, 'XTickMode', 'auto', 'XTickLabelMode', 'auto')
You can replace the call to gca with the axes handle if you have it. Of course you can set the YTickMode, ZTickMode, ect. properties if you applied datetick to the y- or z-axis.
