Aldrea's first two chapters of the Hork-Bajir Chronicles include the date in both andalite terms and human terms - from that we can work out the difference.
Chapter one is andalite year 8561.2, earth year 1966. Chapter two is andalite year 8563.5, earth year 1968. Thus two earth years are equivalent to 2.3 andalite years. Of course, it doesn't tell you when exactly in 1966 or 1968 this happens, so those 2.3 andalite years could be almost anything from just over 1 earth year to just under 3 earth years. This does however tell us that the usual andalite practice is to give the year to one decimal place, and an andalite year must be fairly similar to a human one.
From the Andalite Chronicles, Elfangor talks about hours, days, weeks and years, all seeming to be similar to human time scales - like humans, one can presume they measure time based on days (planetary rotations) and years (planetary orbits of their star), subdivided into hours. Since Elfangor and Aximili do refer to them as "Earth hours" (for instance, in The Invasion) it would seem an andalite hour is not exactly the same as an earth hour.
One interesting thing is that in The Alien the andalite prince Aximili contacts says that he was an aristh under Seerow centuries ago. We know the Seerow's kindness event happened in 1966 from the Hork-Bajir chronicles, so either andalites are very long lived and Seerow was centuries old yet still had a young family, or the andalite meaning of a century is different to the human meaning. (Or a mistake/retcon, of course.)