[Solved] [C#] How to add dashes in Strings?

Hi, I'm having a problem. I need to combine three string values by adding dashes.
But I can't due to this error:

Code:

The '-' operator cannot be applied to operands of type 'string' and 'string' (CS0019)
DDMMYYYY to DD-MM-YYYY
Example: 20022022 to 20-02-2022

My code is:

Code:

string DD = numericUpDown1.Text;
string MM = numericUpDown2.Text;
string YYYY = numericUpDown3.Text;
string BIOSDate = DD-MM-YYYY;
System.Diagnostics.Process.Start("C:\\Windows\\system32\\cmd.exe", "/c date BIOSDate");

Thank you so much.

 

Unfortunately, it didn't. It's about command-line arguments of a program. I need to convert Int32 or String values to Date & Time.

 

Thanks for suggestion. But I get this error either way:

 

I think I forgot to say. It looks like I should not use strings inside brackets. But fixed this error when I use strings outside of brackets. Thanks for help anyways.