cron software package provides process scheduling daemon, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install cron
cron is installed in your system.
Make ensure the cron package were installed using the commands given below,
$ sudo dpkg-query -l | grep cron *
You will get with cron package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Administration Utilities |
| Package | cron |
| Version | 3.0pl1-127+deb8u1 |
| Description | process scheduling daemon |
| Sub Section | admin |