Field.CodeRange Property

Document document = server.Document;
document.BeginUpdate();
document.Fields.Create(document.CaretPosition, "DATE");
document.EndUpdate();
for (int i = 0; i < document.Fields.Count; i++)
{
    string fieldCode = document.GetText(document.Fields[i].CodeRange);
    if (fieldCode == "DATE")
    {
        DocumentPosition position = document.Fields[i].CodeRange.End;
        document.InsertText(position, @" \@ ""M / d / yyyy HH: mm:ss""");
    }
}
document.Fields.Update();

Dim document As Document = server.Document
document.BeginUpdate()
document.Fields.Create(document.CaretPosition, "DATE")
document.EndUpdate()
For i As Integer = 0 To document.Fields.Count - 1
    Dim fieldCode As String = document.GetText(document.Fields(i).CodeRange)
    If fieldCode = "DATE" Then
        Dim position As DocumentPosition = document.Fields(i).CodeRange.End
        document.InsertText(position, " \@ ""M / d / yyyy HH: mm:ss""")
    End If
Next i
document.Fields.Update()