Display error if client System DateTime is back dated in ASP.Net

You can't get the date time form the client PC because the code is executed on the SERVER.

You can use JavaScript to get the current date time from the browser.

After collecting the date time from javascript, set this value in a HiddenField and read on the server and compare with Server DateTime to stop the application from running.

HTML

<asp:HiddenField ID="hfDateTime" runat="server" />
<asp:Button id="btnCheck" Text="" runat="server" OnClick="Check" Style="display: none;" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        string script = "$(document).ready(function () {  $('#hfDateTime').val((new Date()).toISOString().slice(0, 19).replace(/-/g, '/').replace('T', ' ')); $('#btnCheck').click(); });";
        ClientScript.RegisterStartupScript(this.GetType(), "load", script, true);
    }
}

protected void Check(object sender, EventArgs e)
{
    DateTime client = Convert.ToDateTime(hfDateTime.Value);
    DateTime current = GetDateTime();
    if (client < current)
    {
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Client : " + client.ToString() + "\\nServer : " + current.ToString() + "\\nInvalid System DateTime.');", true);
    }
}

public static DateTime GetDateTime()
{
    DateTime dateTime = DateTime.MinValue;
    System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)System.Net.WebRequest.Create("http://www.microsoft.com");
    request.Method = "GET";
    request.Accept = "text/html, application/xhtml+xml, */*";
    request.UserAgent = "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)";
    request.ContentType = "application/x-www-form-urlencoded";
    request.CachePolicy = new System.Net.Cache.RequestCachePolicy(System.Net.Cache.RequestCacheLevel.NoCacheNoStore);
    System.Net.HttpWebResponse response = (System.Net.HttpWebResponse)request.GetResponse();
    if (response.StatusCode == System.Net.HttpStatusCode.OK)
    {
        string todaysDates = response.Headers["date"];

        dateTime = DateTime.ParseExact(todaysDates, "ddd, dd MMM yyyy HH:mm:ss 'GMT'",
            System.Globalization.CultureInfo.InvariantCulture.DateTimeFormat, System.Globalization.DateTimeStyles.AssumeUniversal);
    }

    return dateTime;
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim script As String = "$(document).ready(function () {  $('#hfDateTime').val((new Date()).toISOString().slice(0, 19).replace(/-/g, '/').replace('T', ' ')); $('#btnCheck').click(); });"
        ClientScript.RegisterStartupScript(Me.[GetType](), "load", script, True)
    End If
End Sub

Protected Sub Check(ByVal sender As Object, ByVal e As EventArgs)
    Dim client As DateTime = Convert.ToDateTime(hfDateTime.Value)
    Dim current As DateTime = GetDateTime()

    If client < current Then
        ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('Client : " & client.ToString() & "\nServer : " + current.ToString() & "\nInvalid System DateTime.');", True)
    End If
End Sub

Public Shared Function GetDateTime() As DateTime
    Dim dateTime As DateTime = DateTime.MinValue
    Dim request As System.Net.HttpWebRequest = CType(System.Net.WebRequest.Create("http://www.microsoft.com"), System.Net.HttpWebRequest)
    request.Method = "GET"
    request.Accept = "text/html, application/xhtml+xml, */*"
    request.UserAgent = "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)"
    request.ContentType = "application/x-www-form-urlencoded"
    request.CachePolicy = New System.Net.Cache.RequestCachePolicy(System.Net.Cache.RequestCacheLevel.NoCacheNoStore)
    Dim response As System.Net.HttpWebResponse = CType(request.GetResponse(), System.Net.HttpWebResponse)

    If response.StatusCode = System.Net.HttpStatusCode.OK Then
        Dim todaysDates As String = response.Headers("date")
        dateTime = DateTime.ParseExact(todaysDates, "ddd, dd MMM yyyy HH:mm:ss 'GMT'",
                                        System.Globalization.CultureInfo.InvariantCulture.DateTimeFormat, System.Globalization.DateTimeStyles.AssumeUniversal)
    End If

    Return dateTime
End Function

Screenshot