A [solar] day is the time between noons, which is slightly more than one rotation on our axis. A single rotation is a [sidereal day] — the Wikipedia article has a good animation.

(The ellipse part of our orbit means the length of a solar day isn't consistent, as the rotation required to get back to pointing at the sun isn't the same throughout the year, which is what leads to mean solar time. The the article doesn't want to do ellipse orbits, which is fine… for a moment… but…)

> When the tick comes directly under the sun, that's (solar) noon, and one full rotation is one day.

But this is sort of where if you do you MST (if you have a fixed day length, you are), then when the tick is directly under the sun it won't necessarily be noon. The difference (between MST & solar) is like 17 minutes at its peak. The aliens will be looking at this going "it's uh… close? But off."

I still think "solar time" is a cultural assumption, though I do think there's a high likelihood of aliens sharing that assumption. But one might also imagine a species on a tidally locked world (maybe having grown up in the twilight region) with no concept of "days".

[sidereal day]: https://en.wikipedia.org/wiki/Sidereal_time