Try this:

=If(or(a1=0,b1=0),"",B1-A1-time(8,30,0))

"Xavier" wrote:

hello,

i have 2 calcualte the diffrence from 2 rows and a constant

example
b1-a1 -TIME(8;30;0)

example
08:00 17:00 - 00:30 ok
- -08:30 Problem!!!!! - i did not want to calcualte
dif in this case
08:00 12:00 - -04:30 ok

thanks