When the data are displayed as values I can supress 0. But when I show
the data in date-format 0 is shown as 01-01-1900. Thus the value are
shown again. What am I doing wrong ?
Not telling us what revision of Excel you use and how you are
suppressing zeros in the first place.
I presume that you have a typo and zero formatted as Date actually
displays 01-00-1900 or 00-01-1900 depending on Regional and Language
settings.
In Excel 2003, suppress zero by unchecking the option "Zero Values"
under Tools > Options > View. That works for zero formatted as Date
as well as any other numeric format.
Of course, the real correction might be: do not format that cell as
Date in the first place.
Typically, we see 01-00-1900 because we are doing an operation with
dates (like subtraction) that results in a scalar (simple number), but
Excel decides to format the result as Date because it incorrectly
thinks that is what we want.
We must override Excel's behavior by explicitly setting the format to
Number or some other numeric format other than General.