[Solved] Disable Past dates in HTML5 Calendar Date Picker

You will need to set the minimum date using the min attribute as shown below.

All the dates before the minimum date will be disabled.

HTML

<asp:TextBox ID ="txtDate" runat="server" TextMode ="Date" />

Code

protected void Page_Load(object sender, EventArgs e)
{
    txtDate.Attributes["min"] = "2015-02-15";
}

Screenshot