"And it was at the end of forty years, and Avshalom said to the king: Let me go and pay my vow which I vowed in Chevron" - at the end of forty years since they asked for a king. [Nazir (5, 2). And in Tosafot there, that this was at the beginning of 35 [years] of the kingdom of the house of David, and afterward David reigned another five years, so this was close to Year 2919 or 2920].