UNIX for Advanced & Expert Users

  when              The when field can consist of an interval, a specific
                       time, or both.  If the when field consists of an aster-
                       isk (`*'), log rotation will depend only on the con-
                       tents of the size field.  Otherwise, the when field
                       consists of an optional interval in hours, possibly
                       followed by an `@'-sign and a time in a restricted ISO
                       8601 format or by a `$'-sign and a time specification
                       for logfile rotation at a fixed time once per day, per
                       week or per month.