Improve Performance of Schedule Calculation

In smart_delete_backups(), there is a while loop that could be made more efficient with some simple math:

      // Increment time till we get within one period time span of the period
      // start time. This keeps all the different period starts aligned.
      while ($time < $period_start_time) {
        $time += $period['delta'];
      }

Rather than adding in a while loop, you can subtract the values in the condition and divide by delta to see exactly how many times the delta will be added, then multiply by the delta and add it all at once. This gives the same result, but is noticeably faster:

      if ($time < $period_start_time) {
        $time += ((int) ceil(($period_start_time - $time) / $period['delta'])) * $period['delta'];
      }

Depending on how old the $oldest_file_time is, this can easily shave an entire second off the execution time.