public class MyPdfViewer : PdfViewer
{
    protected override void OnMouseDown(MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            if (Document != null)
            {
                var loc = e.Location;
                int idx = PointInPage(loc);
                if (idx >= 0)
                {
                    var pagePoint = ClientToPage(idx, loc);
                    int zOrder;
                    var ctrl = Document.Pages[idx].GetControlAtPoint((float)pagePoint.X, (float)pagePoint.Y, out zOrder);
                    if (ctrl != null && ctrl.Field.FieldType == Patagames.Pdf.Enums.FormFieldTypes.FPDF_FORMFIELD_TEXTFIELD)
                    {
                        if (FieldIsDateFormatted(Document, ctrl.Field))
                        {
                            MessageBox.Show("Show modaal calendar instead this");


                            //In fact, you should execute the below code after exiting the modal calendar dialog. And this is just an illustration.
                            Document.FormFill.ForceToKillFocus();
                            ctrl.Field.Value = "your own value";
                            Invalidate();
                        }
                    }
                }
            }
        }
        base.OnMouseDown(e);
    }

    private bool FieldIsDateFormatted(PdfDocument doc, PdfField field)
    {
        if (!field.Dictionary.ContainsKey("AA") || !field.Dictionary["AA"].Is<PdfTypeDictionary>())
            return false;

        var additionalAction = field.Dictionary["AA"].As<PdfTypeDictionary>();
        if (additionalAction.ContainsKey("F") && additionalAction["F"].Is<PdfTypeDictionary>())
        {
            var beforeDisplayAction = additionalAction["F"].As<PdfTypeDictionary>();
            var js = PdfAction.FromHandle(doc, beforeDisplayAction.Handle) as PdfJavaScriptAction;
            if ((js.JavaScript ?? "").Contains("AFDate_Format"))
                return true;
        }
        if (additionalAction.ContainsKey("K") && additionalAction["K"].Is<PdfTypeDictionary>())
        {
            var onTypeAction = additionalAction["K"].As<PdfTypeDictionary>();
            var js = PdfAction.FromHandle(doc, onTypeAction.Handle) as PdfJavaScriptAction;
            if ((js.JavaScript ?? "").Contains("AFDate_Keystroke"))
                return true;
        }
        return false;
    }
}