I built a small data file to demonstrate datetick.  Data is plotted for October 10, 2012 at hours from 0100 to 1300.  Here is the data:

2012 10 9 01 00 00 47
2012 10 9 02 00 00 46.5
2012 10 9 03 00 00 45
2012 10 9 04 00 00 45.2
2012 10 9 05 00 00 45.3
2012 10 9 06 00 00 46
2012 10 9 07 00 00 46.5
2012 10 9 08 00 00 47.6
2012 10 9 09 00 00 48.2
2012 10 9 10 00 00 49.8
2012 10 9 11 00 00 51.2
2012 10 9 12 00 00 53.5
2012 10 9 13 00 00 54.7
2012 10 9 14 00 00 56.9

The following code generates a graph that shows
m=load("temp20121009.dat");
t = datenum(m(:,1:6));

this generates the following Julian dates.  The differences look fine, an hour is 0.04166666
   735151.041666667
   735151.083333333
   735151.125000000
   735151.166666667
   735151.208333333
   735151.250000000
   735151.291666667
   735151.333333333
   735151.375000000
   735151.416666667
   735151.458333333
   735151.500000000
   735151.541666667
   735151.583333333

However, when plotted:
plot(t, m(:,7))
the numbers generate odd spacing.  They are all equally spaced.
Seems like a bug in the autoselection of tick locations.

This problem does not happen with integral number of days.