Ruizong handed the throne to his heir… (712) — the same event in New Book of Tang and Zizhi Tongjian
New Book of Tang: Ruizong handed the throne to his heir Longji and became Taishang Huangdi (accession of Xuanzong).; Zizhi Tongjian: In August 712…
What each source says
Ruizong handed the throne to his heir Longji and became Taishang Huangdi (accession of Xuanzong).
庚子，即皇帝位
Location: 新唐書 卷005 睿宗·玄宗 先天1年8月
In August 712 (Gengzi day), Emperor Xuanzong ascends the throne and honors Ruizong as Taishang Huang.
八月，庚子，玄宗即位，尊睿宗為太上皇。
Location: 권210 712년 항