Cron as a tiny shell script (GNU tools required)

minicron.bash

function minicron {(
  set -o errexit -o pipefail -o nounset
  spec="$1" ; shift
  cmd=( "$@" )
  present="$(date -u +%FT%TZ)"
  target="$(date -u +%F)T$spec:00Z"
  next_target="$(date -u -d tomorrow +%F)T$spec:00Z"

  if [[ $present > $target ]]
  then target="$next_target"
  fi

  echo "Awaiting: $target" >&2

  while sleep 60
  do
    present="$(date -u +%FT%TZ)"

    if [[ $present > $target ]]
    then
      next_target="$(date -u -d tomorrow +%F)T$spec:00Z"
      if ("${cmd[@]}")
      then
        target="$next_target"
        echo "Awaiting: $target" >&2
      fi
    fi
  done
)}