Adds two instances of Duration together.
Assembly: PresentationCore (in PresentationCore.dll)
Return ValueType: System.Windows.Duration
If both instances of Duration have TimeSpan values, this method returns the sum of those two values. If either value is set to Automatic, the method returns Automatic. If either value is set to Forever, the method returns Forever.
If either t1 or t2 has no value, this method returns null.