In old time both astronmoers and astrologers were called
Jyotishi and only now we use term Khagol Shastri for Astronomer.
Guru ji did know alamanac or Panchang fro the use of time
keeping like the days of Gurpurabs or other such festivals. Then
during Vaisakhi 1699 it is possilbe thaT Guru did took use of
Astronomers to decide the day for gathering why? Well we must
understand that Birth of First Master coincide with Vaisakhi and
his Birth on Kartik Purnima(luner Month of Katik whith Vikrami
era and fullmoon day) is nothing more then pagan ritual of
tubarhi being celebrated on the same day like a german festival
on 25 Dec beacem Birthday of Christ. So it is possilbe that Guru
used Astronomers and alamanc for one more reason. As sun moves
with respect to earth and we get to know as it enter various
zodiac(Rashi or relativly fixed arrangements of stars).this
thing made Arybhatta discover that earths rotates around
sun.Reference:: Sikh Philosophy Network
http://www.sikhphilosophy.net/sikh-sikhi-sikhism/3219-horo