Try
var ts = TimeSpan.Parse(stringTime);
With a newer .NET you also have
TimeSpan ts;
if(!TimeSpan.TryParse(stringTime, out ts)){
// throw exception or whatnot
}
// ts now has a valid format
This is the general idiom for parsing strings in .NET with the first version handling erroneous string by throwing FormatException and the latter letting the Boolean TryParse give you the information directly.