You've got to create datenum array to match to plot against for datetick to work correctly. Or you need to plot the x axes using datetime values instead. It depends on how the plot data are created to determine which is easier...if you just have numeric data at each time point, then datetime is trivial--if there's a curve fit against time in seconds using fit as in (I'm presuming) the other very similar Q? by a different monikor, then the second axes is probably easier...
From here you can modify axes limits and other properties as desired. Just remember have to keep the two in synch as to how many points are shown and that the limits of how many seconds are shown in the first are those that correspond to the correct time in the second.
If you have a way to generate the y data versus a datetime; it's the simpler solution as would only have the one axes and the dates would be incorporated directly.