sql server 2008 for adding date

Difficult to be sure with that little info, but the most likely reason is that the PC is set to a locale where the date format is confusing SQL when you try to add it to the database as a string.
 
Instead, convert the TextBox.Text property to a DateTime value using the Parse or TryParse methods, and pass the DateTime itself to SQL as a parameter instead. You should not be passing TextBox contents to SQL directly anyway - it is a recipe for SQL Injection Attacks which can damage or destroy your database. Always use Parametrized queries to avoid this, and your Date problems will disappear as well.