(job '(next-hour-from (next-day) '(16)) "my-program")
my-program every day at four o'clock in the
afternoon. However, if mcron is started with this script at midday,
the first time the command will run will be four o'clock tomorrow;
today's appointment will be missed (one time only).
The correct way to specify this requirement is simply
(job '(next-hour '(16)) "my-program")