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According to rclcpp::timer docs, std::chrono::steady_clock is used,

using   WallTimer = GenericTimer< CallbackType, std::chrono::steady_clock >

Quoting from std::chrono::steady_clock docs,

Class std::chrono::steady_clock represents a monotonic clock. The time points of this clock cannot decrease as physical time moves forward and the time between ticks of this clock is constant. This clock is not related to wall clock time (for example, it can be time since last reboot), and is most suitable for measuring intervals.

So to answer your question, Yes, it is guaranteed never to jump backwards (or forwards) due to midnight, daylight savings time, user changing the system clock, clock adjustments via NTP, etc