midwesterner – The Rambam is Hilchos Kiddush Hachodesh explains how the calculation of the molad is calculated from Brias Haolam (this is one of the reasons why Rav Schwab ZT”L was chozer, because according to his calculation the cheshbon of the molad wouldn’t fit).
It does indeed appear from the Rambam’s method of calculation that there was a continuous use of the years since Brias HaOlam, regardless if this count was used in every day use (where minyan shtaros was used).