Jeremiah 39:1

39:1 King Nebuchadnezzar of Babylon came against Jerusalem with his whole army and laid siege to it. The siege began in the tenth month of the ninth year that Zedekiah ruled over Judah. ¹ 

[39:1]  ¹ sn 2 Kgs 25:1 and Jer 52:4 give the more precise date of the tenth day of the tenth month of the ninth year which would have been Jan 15, 588 b.c. The reckoning is based on the calendar that begins the year in the spring (Nisan = March/April).