Monotonic timers relative to different starting points

Settings and their starting points

Setting	Meaning
OnActiveSec=	Defines a timer relative to the moment the timer unit itself is activated.
OnBootSec=	Defines a timer relative to when the machine was booted up. In containers, for the system manager instance, this is mapped to OnStartupSec=, making both equivalent.
OnStartupSec=	Defines a timer relative to when the service manager was first started. For system timer units this is very similar to OnBootSec= as the system service manager is generally started very early at boot. It's primarily useful when configured in units running in the per-user service manager, as the user service manager is generally started on first login only, not already during boot.
OnUnitActiveSec=	Defines a timer relative to when the unit the timer unit is activating was last activated.
OnUnitInactiveSec=	Defines a timer relative to when the unit the timer unit is activating was last deactivated.