SlackBuildsOrg/network/radicale

Radicale (Calendar Server)

The Radicale Project is mainly a calendar server, giving local and
distant accessess for reading, creating, modifying and deleting
multiple calendars through a simplified CalDAV protocol. Data can be
encrypted by SSL, and their access can be restricted thanks to
different authentication methods.

You need a "radicale" user and group prior to building radicale.
Something like this will suffice for most systems:

  groupadd -g 375 radicale
  useradd -u 375 -d /var/lib/radicale -s /bin/false -g radicale radicale