Unfortunately, we don't know the precise date Rosetti began his service at Wallerstein. One can, however, deduce the year and possibly even the month from evidence found in court records of a later period. In a petition to the prince dated May 19, 1786, Rosetti refers to his “thirteen years of service” placing his date of hire in 1773. This reading is further supported in a document from 1789, Rosetti's last year at court, in which he prefaces a request for release from his position by stating that “for sixteen years I have dedicated the best years of my life to your most serene Highness for a yearly salary of 402 gulden.” Even more specific is a petition directed to the prince on December 23, 1775. Here Rosetti speaks of “two and a quarter years of service,” establishing his arrival at court some time around November 1773. This is confirmed by a receipt dated November 30, 1773, for lodging Rosetti at the expense of the prince in the Hohenaltheim inn operated by Johann Bergdolt.

Although already an experienced professional musician, Rosetti was hired as a servant in livrée and paid accordingly. On the payroll his name is found among the house servants earning a monthly salary of 12 gulden, totaling 144 gulden annually. This amount was supplemented by 3 gulden for shoes and stockings. Although Rosetti's primary duty was to play violone in the orchestra, in his first years at court he would have had other responsibilities as well.