string wrongType = "This is not a date";
command.Parameters.Add("@Date", SqlDbType.DateTime).Value = wrongType;

string wrongType = "This is not a date";
DateTime rightTyped;

if(DateTime.TryParse(wrongType, out rightTyped))
{
    command.Parameters.Add("@Date", SqlDbType.DateTime).Value = rightTyped;
}

string wrongType = "This is not a date";
DateTime date;
if(DateTime.TryParse(wrongType, out date))
{
    // staff when string converted
}

string wrongType = "This is not a date";

DateTime rightType = ...

command.Parameters.Add("@Date", SqlDbType.DateTime).Value = rightType;