Dates on contour plots

Dear all,

  I new to matplotlib and I need some help with a very simple procedure.
I've been successful to plot dates on the x-axis with the "plot"
command. However, I've been unable to use dates with the contour and
contourf commands.

The command I'm using is:

CS = contourf(time, depth, dens)

Time x-axis shows only the "number" instead of the dates like --

Thanks for any help. Filipe