Functor min/max for formatted Date object is broken.
Is it intentional and i have to use self defined groupmethod like one below or is it a bug?
It is the expected behavior, default operations work for numbers only.
Current version provides the invalid result for Date objects, the updated one ( will be released in few weeks ) will ignore the non-numerical values from min/max/sum operations.
The solution with custom method is the correct one and will work with the updated version as well.