The set of days of the month when this job should run.
Bits beginning with 'd' specify a day relative to the
first day of the month. Bits beginning with 'r' specify
a day relative to the last day of the month. Bit 'r1'
is the last day of the month, bit 'r2' is the second to
last day of the month, etc.
Some devices may allow only a single day to be specified
in the bitmap.