How to make excel ignore errors when summing up cells

Hello,

How can I make excel sum up a colum, and ignore cells shown as #I/T?
This error sign might not be the same in the english version of excel.

If I try to sum up cells with the error value, the feedback will also be #I/T.

I have uploaded a screenshot to illustrate the problem for You

(The reason for the faulty cells is that the date does not contain any values)

thanx! =)

Faulty formula is now; =(SUM(B307:B316))