if (!string.IsNullOrEmpty(SessionDictionary.GetValue("UserDetails", "ExpiryDate")))
{
    DateTime ExpiryDate = DateTime.ParseExact(SessionDictionary.GetValue("UserDetails", "ExpiryDate"), "dd mmm yy", null);                      
    strDate = sitedata.FormatDate(ExpiryDate, TridionDateFormat.ShortDate);
}
else
{
    strDate = "-";
}
	
DateTime.ParseExact(..., "dd/MM/yyyy HH:mm:ss", CultureInfo.InvariantCulture);
	
DateTime ExpiryDate = DateTime.ParseExact(SessionDictionary.GetValue("UserDetails", "ExpiryDate"), "dd/MM/yyyy hh:mm:ss", null)
	
DateTime ExpiryDate = DateTime.ParseExact(SessionDictionary.GetValue("UserDetails", "ExpiryDate"), "G", null)