Dear maintainer:
The timeshift source code
(https://github.com/linuxmint/timeshift/blob/2518b06/src/Core/Main.vala#L342)
checks for the crontab binary before running. Although I did not plan to use
the (fairly limited) cron management dialog in timeshift, it refused to startup
and complained that I did not install a cron daemon in my minimal system. I
hope the upstream developers could make the rsync and crontab dependencies
optional in the future, but meanwhile the cron-daemon virtual package should be
declared as a required dependency for all the currently packaged versions in
Debian.