The only assumption is that analog means a gauge of some sort with a pointer or pointers moving between symbols, rather than a series of symbols that change for digital. I wouldn't have any problem with assuming a circular gauge (because we calculate periodic things using circle math), but it's not necessary.
That's kind of a big assumption, IMO. Why does it need symbols? Perhaps a hypothetical alien species has an analog clock that keeps track of times with shades of color. Or audio tones. Or it uses a gauge, but the gauge vibrates and the vibrations mean something.
Combine that with the fact that their time system might not be based on a fairly regular rotation of the planet. Perhaps they value some other less regular measurement more. Or perhaps their planet does not have regular rotation. How might a time system evolve and be portrayed in those cases? How can you be sure that an analog display would be easier to interpret than a digital one?
Similar to how a 12-hour analogue clock does not necessarily imply that we actually have 24 hours in our days, an alien analog clock could potentially be vastly different than the same alien's digital clock.
