What do animals understand about time?
More than the traditional view allowed and less than the popular one assumes — animals clearly track duration, rhythm and sequence, while the ability to mentally travel to a specific past or future is far harder to demonstrate.
What is well established:
Circadian rhythms, internal clocks synchronised to light that govern sleep, feeding, hormones and activity across essentially all animals. This is why a dog expects dinner before you reach for the bowl.
Interval timing — measuring elapsed durations of seconds to minutes — demonstrated across many species.
Anticipation of regular events, learned through association. A cat waiting at the door before a car arrives is more plausibly responding to routine, sound and scent cues than to clock reading.
Seasonal timing, driven by day length, governing migration, breeding and hibernation.
Where it gets interesting: episodic-like memory. True episodic memory means recalling a specific event — what happened, where, and when. Scrub jays, which cache food, have been shown to recover perishable items sooner than non-perishable ones after varying delays, implying they track how long ago each item was stored. Similar evidence exists in rats, some primates and cuttlefish. Researchers describe this as episodic-like memory, carefully, because demonstrating the subjective experience of remembering is not possible.
Future planning is harder still. Some corvids and great apes select and save tools for use later, in contexts designed to exclude immediate reward — which is among the strongest evidence for planning beyond current needs.
What this means for pets: dogs and cats almost certainly do not experience "you were gone for six hours" as a duration in the way we would. Separation distress is about the absence itself, not the elapsed time, which is why greeting behaviour is not a reliable guide to how long you were away.
Why it is hard to study. Animals cannot be asked, so every experiment must exclude simpler explanations — associative learning, familiarity, and cues the experimenter did not intend.