- // Touch must return a non-nil error unless it can guarantee
- // that a future call to Mtime() will return a timestamp newer
- // than {now minus one second}.
+ // If invoked at time t0, Touch must guarantee that a
+ // subsequent call to Mtime will return a timestamp no older
+ // than {t0 minus one second}. For example, if Touch is called
+ // at 2015-07-07T01:23:45.67890123Z, it is acceptable for a
+ // subsequent Mtime to return any of the following:
+ //
+ // - 2015-07-07T01:23:45.00000000Z
+ // - 2015-07-07T01:23:45.67890123Z
+ // - 2015-07-07T01:23:46.67890123Z
+ // - 2015-07-08T00:00:00.00000000Z
+ //
+ // It is not acceptable for a subsequente Mtime to return
+ // either of the following:
+ //
+ // - 2015-07-07T00:00:00.00000000Z -- ERROR
+ // - 2015-07-07T01:23:44.00000000Z -- ERROR
+ //
+ // Touch must return a non-nil error if the timestamp cannot
+ // be updated.