>Thank you for both replies. What happens is that if the value of @TargetDate is NULL, all works.
>Or if the value of @TargetDate from the XML type is American format (that works for many customers), all works.
>But when I set the value in the ASP.NET application to the XML element TargetDate, I get error.
>Why I do it this way is a long story. But it works for many years.
>Now I need to figure where it breaks for the British date.

"if the value of @TargetDate from the XML type is American format" - That is what I am saying, in the XML the format for a DateTime is fixed, regardless you are living in US or Turkey, or choose any place you like. It is like:

2019-02-28T19:00:00+03:00

I don't understand what does XML datetime have to do with ToShortTimeString(). Are you sure your XML is defining a DateTime type?