.NET Framework 4.6 and 4.5
Subtracts the value of one instance of Duration from another.
Assembly: PresentationCore (in PresentationCore.dll)
Return ValueType: System.Windows.Duration
If both instances of Duration have values, an instance of Duration that represents the value of t1 minus t2. If t1 has a value of Forever and t2 has a value of TimeSpan, this method returns Forever. Otherwise this method returns a null reference (Nothing in Visual Basic).