Gurfateh
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-horoscopes-janam-kundli.html
But due to movement of earth with respect to sun and suns being visible in differant zodiac gives us one more inforamtion.Declation of soler Angle in either of Hemisphere and its change causes season to change.
So may be Guru called all Sangat(holy Company) to come after they were free from thier daily jobs of crop protection as it was the time of Harvest and after harvest Singhs could have got a good lot of resources to raise fund for WAR.
Then while leaving Chamkaur's fortess,Guru and his Sikhs used the stars to gauge the way.
and still in survey and in Army Almanac and steller positions are used to get way in remote Area to get longitude or latitude of Area even when GPS fails.GPS is artifiaicla sattlite which has its own almanac and does the same.
So It can not be ruled out that Guru did used astronomy but NEVER NEVER did they used Astorlogy as some foolish hindutva people say(they are sons of those people who called Arya Bhatta mad fro his saying that Earth rotate aroung the sun and such tendency of uniting Astronomy with Astrology ruined the acint Vedic Era astrolnomy).
As in Japu Ji saihb itself it is writtan.
Huaqqami Ander Sabh Ko Bahar Huqam Na Koi,Nanak Huqqami Jai Bujhe,Hau mai Kahe Nan Koi.
All universe works under will of God and if someone understand it then no ego is left.
So Horoscope,which tells by pseudo scince that by the position of stars or plannet our life is gooing to be effected and by particular riltaism or stone we can undo damge has no place in Gurmat.As If Akal want something to happen then no object star or Plannet can prevent it.Rather all celestial objects move by will of Akal and no one knows it but Akal so why should we try to shamm with astrology or Horoscope.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=3219
Brother Balbeer Singh Ji Das hope you have got the point.