Why is there no [`DateTime.Add(DateTime)`](https://learn.microsoft.com/en-us/dotnet/api/system.datetime.add?view=netstandard-2.0) overload when there is a [`DateTime.Subtract(DataTime)`](https://learn.microsoft.com/en-us/dotnet/api/system.datetime.subtract?view=netstandard-2.0) overload?

Given the similarity of Add and Subtract operations, I would expect they would have the same overloads. Why might this not be the case?