The expression for the relative cutoff timestamp was
`-self.maximumNumberOfLogFiles * kDayInterval`, but
'maximumNumberOfLogFiles' is an unsigned integer, so this turned into
"some day very far in the future" and immediately deleted all
extension logs. The new expression, `self.maximumNumberOfLogFiles *
-kDayInterval` works because 'kDayInterval' is an NSTimeInterval, a
floating-point value.