c# - System.FormatException: String was not recognized as a valid DateTime -

i using ajax calender extender textbox, , want disable past dates, user not able select those, less today's date. code follows

in .aspx page

<asp:textbox id="txtfromdate" runat="server" ontextchanged="txtfromdate_textchanged"        autopostback="true"></asp:textbox>      <asp:calendarextender id="calendarextender1" runat="server" targetcontrolid="txtfromdate">         </asp:calendarextender>         <asp:toolkitscriptmanager id="toolkitscriptmanager1" runat="server">         </asp:toolkitscriptmanager> 

and on .cs page

protected void txtfromdate_textchanged(object sender, eventargs e)     {         datetime dt1 = datetime.parse(txtfromdate.text);         if (dt1 < datetime.now)         {             //response.write("you can not choose date earlier today's date");             txtfromdate.text = null;         }      } 

but getting following error:
system.formatexception: string not recognized valid datetime.
, there way can make date unclickable user cant choose them using startdate , enddate attributes, or other means, tried again got error these not supported. appreciated.

try if date's format "yyyy/mm/dd":

string[] date = typeddate.text.split(new char[] { '/' }); datetime dy = new datetime(int.parse(date[0]), int.parse(date[1]), int.parse(date[2]));