Inserting into a SQL DB

Hi

I am trying to insert a record into a sql db but keep getting the following error:

CS0029: Cannot implicitly convert type 'System.DateTime' to 'string'

the code behind the aspx page is the following: 
public void btn_SubmitSIMDetails_OnClick(object sender, EventArgs e)
{
if (tbx_PhoneNumber.Text == "")
{
MessageBox("You Must Provide a Vaild Phone Number");return Go to the complete details ...