countFiles := make(map[string][]string) // expiry date string->filenames
	earliest := make(map[string]time.Time)  // earliest begin time for any counter
	for _, f := range todo.countfiles {
		begin, end, err := u.counterDateSpan(f)
		if err != nil {
			// This shouldn't happen: we should have already skipped count files that
			// don't contain valid start or end times.