Samuel and United Monarchy

1 Sam 4:18 Eli judged Israel 40 years.
1 Sam 4:15 Eli was 98 years old.

Samuel was born perhaps ca. 1100 BC.

1 Sam 13:1 "Saul was 30 years old when he began to reign, and he reigned for 42 years over Israel." [textual issues]
Acts 13:21 "Then they asked for a king, and God gave them Saul, son of Kish, of the tribe of Benjamin, for forty years."

2 Sam 2:11 "The time that David was king in Hebron over the house of Judah was seven years and six months."
2 Sam 5:4–5 "David was 30 years old when he began to reign and he reigned 40 years. In Hebron he reigned over Judah seven years and six months, and in Jerusalem he reigned 33 years over all Israel and Judah."

According to Eugene Merrill, Ish-bosheth was about 35 years old when his father Saul died, but was installed as king when he was 40. During the approximate 5 year interregnum, it seems Abner acted as ruler. The Bible does not mention this. It is inferred by the chronological reconstruction and by Abner's role in appointing Ish-bosheth as Saul's successor (2 Sam 2:8–9).

2 Sam 2:10 "Saul’s son Ish-bosheth was 40 years old when he began to reign over Israel, and he reigned for two years. But the house of Judah followed David."

2 Sam 5:1–5 David's reign from Jerusalem appears to begin at the same time he is anointed king over all Israel, after Ish-bosheth's death.

2 Sam 2:11 "The time that David was king in Hebron over the house of Judah was seven years and six months."
2 Sam 5:4–5 "David was 30 years old when he began to reign and he reigned 40 years. In Hebron he reigned over Judah seven years and six months, and in Jerusalem he reigned 33 years over all Israel and Judah."

2 Sam 15:10–13
2 Sam 3:2–3; 1 Chr 3:1–2 Absalom was born to David while David lived in Hebron, i.e. ca 1010–1003 BC.

2 Sam 20:10 "So all the men of Israel withdrew from following David and followed Sheba son of Bichri, but the men of Judah kept close to their king from the Jordan to Jerusalem."