Note that a TimeSpan does not have a format . It is stored in some internal representation, which is not at all like 00:10:00.
hh:mm:ss , TimeSpan String, . , - , - . " " - TimeSpan - TimeSpan.
String, String.Format, V4Vendetta, TimeSpan.ToString ( .NET 4):
string formattedTimespan = ts.ToString("hh\\:mm");