My turn ^_^
CultureInfo provider = CultureInfo.InvariantCulture;
string clockFormat = "hh:mm";
string clockStr = txt1.Text;
string minutesStr = txt2.Text;
double minutes;
DateTime clock;
TimeSpan time;
if(!DateTime.TryParseExact(clockStr,clockFormat,provider,DateTimeStyles.None, out clock))
clock = new DateTime(0,0,0);
TimeSpan clockTime = clock.TimeOfDay;
if (double.TryParse(minutesStr, out minutes))
minutes = 0.0;
time = TimeSpan.FromMinutes(minutes);
DateTime totalTime = clock.Add(time);
TimeSpan totalTimeSpan = clockTime.Add(time);