A note on this spec:

`#to_formatted_s` exhibits incorrect (or at least very surprising) behavior with datetime values.  It uses `strftime` in a mode that converts its input to local time and then generates a formatted string.

Aside from being odd, it makes writing a correct example infeasible: you cannot make any assumptions about the timezone of the system running your test, nor can you realistically require that running the spec suite require that the user running the suite have privileges to set the system timezone.

However, that's another issue for another day.