compare txtDate and systemDate

Your code should be like below. But you need to save the input date some where. So that you will get the date when you input the value in the textbox. Here i have saved in a label.

Code

string nextDay = string.Empty;
DateTime date = Convert.ToDateTime(txtDate.Text);//Texbbox value.
lblEntryDate.Text = DateTime.Now.ToShortDateString();//Save entry date.
DateTime entryDate = Convert.ToDateTime(lblEntryDate.Text);
DateTime today = DateTime.Now;
int dateDiff = (today - entryDate).Days;
nextDay = date.AddDays(dateDiff).ToString();
txtDate.Text = Convert.ToDateTime(nextDay).ToShortDateString();
if (txtDate.Text == today.ToShortDateString())
{
    //program run
}
else
{
    //show message please check system date
}

HTML

<asp:Label ID="lblEntryDate" runat="server" Visible="false" />
<asp:TextBox runat="server" ID="txtDate" Text="2016/04/06" />