FTPHost.set_time_shift(new_time_shift) must invalidate (i. e. clear)
the stat cache. Otherwise the cache contains modificiation times that
were calculated under assumption of the previous time shift value. These
times in the cache would be wrong with respect to the new time shift.
This issue was solved in d1ac5ecc064cbd076f896cf0d5e7aedecc4bc816.