Rashi citing Temurah 14b: It is from when Yisrael requested a king ¹ ; there was a rebellion and descent in the kingship.

Malbim citing Mahari, R. Yeshayah: It is from [the start of] David's kingship ² . Malbim - i.e. close to the end of 40 years. "Mi'Ketz" means close to the end, e.g. "mi'Ketz Sheva Shanim Ta'aseh Shemitah" (Devarim 15:1). It was year 37 or 38 of David; it was likely that he would make Shlomo king in his lifetime.

Radak: When in Geshur, he vowed to bring a Korban in Chevron.

Rashi (from Temurah 14b): Chevron was the best place to get lambs ¹ .

Radak Even though the Mishkan was in Giv'on, Bamos were permitted, so he could offer anywhere (Temurah 14b). He chose to rebel in Chevron, for there his father became king. Malbim - it is parallel to Yerushalayim, where David now reigned. From there, Avshalom will draw Yehudah after himself.