Humans conceive of time through the concept of a circle apportioned into 360 degrees and subdivided into 60 parts, anchored against the speed of the rotation of the planet. This is how we perceive it and then you can translate that into how it works in TES (Magnus revolves around Nirn), but the concept of using a circle to spin 'round and 'round remains, even if you're viewing it as a wheel progressing forward/backward or a spiral, circling and circling the same point, but changing it's 3-dimensional position. You can point your finger at whatever it is, the point is that we are obsessed with an ideal, a circle, as our base for measuring the progress of time.
So, how would sentient Trees, whose obsession would be more towards growing measure time? At first glance, you might say "Tree-Rings", but they aren't going to cut themselves apart just to count the rings they grew. You could argue a foliation/defoliation cycle, but Black Marsh is supposed to be a swamp and every swamp I know (In RL) has deciduous trees keep their leaves all year.