what we need to know that what is the calender followed at Andal's time. That will help to confirm the period.
The present astrologers follow not only many ayanamsa like BV Raman and Lahari and KP.
There is another issue of Vakya panchang and Thirukkandia Panjang. Both schools claim they are right.
I raised this doubt when recently Saturn, rahu, Ketu and Jupiter changed consequently and with two different panchangs giving different dates - a person can end up having two totally different horescopes with variation in the houses for 4 planets.
as somebody pointed out, astrology is a 100% science.. it falters nowadays because of ill informed/misinformed astrolegers and mis interpretation..
for instance, some online site gave my jaadhagam as "with Chevvai dosham".. but when verified with lot of other astrolegers it was told that some "vakram of sevvai" was there and it cannot be called as "chevvai dosham" and hence no "chevvai dosham" in my jaadhagam..
all these fun was during the search by my parents for a bride for me....