Try like this..
public void Create<T>(T obj)
{
var created_date = typeof(T).GetProperty("created_date");
var modified_date = typeof(T).GetProperty("modified_date");
created_date.SetValue(obj, DateTime.Now, null);
modified_date.SetValue(obj, null, null);
}
public void Modify<T>(T obj)
{
var created_date = typeof(T).GetProperty("created_date");
var modified_date = typeof(T).GetProperty("modified_date");
created_date.SetValue(obj, created_date.GetValue(obj, null), null);
modified_date.SetValue(obj, DateTime.Now, null);
}