the date value of series is incorrectly displayed

I'm drawing a 3d graph with 2 series of data. The graph is displayed as 2
separate rows of bars behind each other. In Excel 2007 the type is Colum in
the first row to the far right.

1. series is a units of work. y -axis displayed in units of Thousands
2. series is a date when the units were done. x -axis.

If I hover over the the date bar in the 2nd bar row I can see all the right
values
including the right date.

the table is:

10000 1.11.2009
10000 3.11.2009
50000 10.11.2009 (in fact todate() function

All the dates are displayed as 9.02.1900 if I try to display them as Data
labels.

The dates are displayed correctly if displayed as x -axis labels.

Any ideas to correct this?