Quennar, surnamed Onótimo, was a Loremaster of Aman. He wrote extensively about the reckoning of time among the Valar. His great work the Yénonótië was cited by Pengolodh in the Annals of Aman, particularly his writing explaining the length of the Valian year and its relation to the waxing and waning of the Two Trees. Quennar also calculated the duration of Valian years compared to Years of the Sun. Aside from his work on reckoning that Pengolodh cited, almost nothing is known about Quennar's writings or his life in general.
Quennar's first name is a bit mysterious; the first part seems to incorporate the Quenya word for 'person', quen. The second part of his name might be related to nár, 'flame'. Onótimo means something like "Reckoner", originating from the verb onot- 'count up' and the masculine agental suffix -mo.
Other Versions of the Legendarium
Originally, "Quennar i Onótimo" was to have been the author of the Annals of Aman as a whole, having learned and borrowed much writing from Rúmil. Eventually, Tolkien attributed the Annals to Rúmil himself, while adding "quoth Pengolodh" to the end of the section ascribed to Quennar. This probably means that Pengolodh quoted Quennar's work in his enlargement of the Annals, which presumably took place after the Exile of the Noldor. Since Quennar's writing discusses the Years of the Sun, which did not exist while the Noldor still lived in Aman, it is most likely the case that in the final version of events as envisioned by Tolkien, Quennar wrote the Yénonótië after the Exile and was thus a Noldo who came to Middle-earth.