[bheadmaster]

Portable means easy to make it work while changing the underlying system or its implementation details - where the underlying system refers to architecture, OS, API or any other dependencies needed to run the program.

In context of systemd, systemd isn't portable because it highly depends on Linux API and thus cannot be ported on any other UNIX-like (or unlike) OS.

In context of systemd timers, systemd timers aren't portable because there isn't as single alternative implementation that can interpret systemd timer unit files.