DateTime in MSCRM

How to format string to write in CrmDateTime attribute:

string startdate; // - can be a parameter in method

opportunity oop = new opportunity();

opp.new_startdate = new CrmDateTime();

if(startdate!="")
   opp.new_startdate.Value = (Convert.ToDateTime(startdate)).ToString("yyyy-MM-ddTHH:mm:ss");
else
   opp.new_startdate.Value = DateTime.Today.ToString("yyyy-MM-ddTHH:mm:ss");