, I do not know which dude are you talking about. Prajapati is 'Yajna' (Orion, Mrigashiras).
"In Rig. x. 192.2, Samvatsara or the year is said to rise out of the ocean, the place where Vritra was killed (Rig. x. 68.12). Prajapati, as represented by Orion, may also be naturally supposed to commence the year when the vernal equinox was in Orion. Rudra killed Prajapati, and as I have shown before, Prajapati, Samvatsara and Yajna were convertible terms. Rudra therefore killed Prajapati or Yajna at the beginning of the year; and Yajna also meant sacrifice. Rudra was therefore naturally believed to have killed the sacrifice thus giving rise to the Puranic legends of Rudra routing the sacrifice of Daksha. At the end of the Sauptika Parva in the Mahabharata we are told that "Rudra pierced the heart of Yajna or Sacrifice with an arrow. Thus pierced the Sacrifice, with fire, fled away in the form of an antelope and having reached the sky, there shines in that form, followed Rudra." Thus it was that Rudra acquired the title of Sacrifice-breaker. In the Tandya Brahmana vii.2.1, the death of Prajapati is, however, spoken of as voluntary.
In Taitt. Br. iii. 9. 22.1, he is said to have assumed the form of Yajna and given himself up to the Devas to be sacrificed. The Devas killed him on their morning, and so every one should similarly perform the Ashvamedha sacrifice at the beginning of the year. One can now understand what the meaning of these stories is. They refer to the death of Prajapati by Rudra at the beginning of the year; and thus it was that Yajna, meaning the year was sacrificed by means of Yajna or Prajapati, Rig. x. 90.16., where we are told that Gods sacrificed Yajna by Yajna, but this (human sacrifice) was an old (out of date) practice, may also be similarly interpreted. I cannot say which of these legends is older, whether that of Prajapati sacrificing himself, or of Rudra killing him at the beginning of the year. But whichever of these be older, it does not affect our present question. Both of them indicate that Prajapati once commenced the year and that he either willingly allowed himself to be sacrificed or was killed by Rudra at that time."
"ORION or Researches into the Antiquity of Vedas" by B.G.Tilak, Chapter 6 - Orion and His Belt, Page 124 onwards.
Can we, insignificant fellows, match the effort and scholarship of Lokmanya Bal Gangadhar Tilak? How many books you find referred here? Have we read even one hundredth of what he has mentioned?
ps - The year began with Orion/Mrigashiras on the day of vernal equinox during the period 4,000 BC to 2,200 BC, when it was changed to Krittika; and later to Ashwin (followed currently) around 600 AD following Bhaskara. It needs to be corrected now as the asterism today on the day of vernal equinox is Revati (Pisces).