[dist-epoch]

> mass-spring system is periodic

I don't pretend to understand this stuff, but wouldn't a real mass-spring system slowly stop, due to friction, air resistance, heat dissipation, ...? So a real system wouldn't be periodic.

[sidlls]

Periodic doesn't mean perpetual, perfect, constant periodic motion (in general).

[consilient]

Yes, but they're talking about an idealized harmonic oscillator, not a physical mass-spring system.