For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:weberho:UNSTABLE:tine20/openSUSE_Tumbleweed/home:weberho:UNSTABLE:tine20.repo
zypper refresh
zypper install tine20-calendar
For openSUSE Leap 15.2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:weberho:UNSTABLE:tine20/openSUSE_Leap_15.2/home:weberho:UNSTABLE:tine20.repo
zypper refresh
zypper install tine20-calendar
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:weberho:UNSTABLE:tine20/15.3/home:weberho:UNSTABLE:tine20.repo
zypper refresh
zypper install tine20-calendar