Why was Milton Young President Pro Tem for one day?

Milton Young (R-ND) is listed as President Pro tem of the US Sentate for just one day, December 5, 1980. Why did he serve only one day? Did it have something to do with his length of service and upcoming retirement? An acknowledgement of the coming Republican majority which would take office Jan 3 1981?

SWAG: With the incoming Republican majority, Strom Thurmond would become the new President pro tem, as Senator Young was retiring. By the time the new Senate was sworn in in early January, he would be gone. So, it was probably just a tip of the hat from this exclusive club to one of their own for his years of public service. Coincidentally (or not), December 5 was Thurmond’s birthday.

Similarly, a dying Hubert Humphrey was given the post of “Deputy President pro tem” a few years earlier. I don’t know that anyone else has had that post. If so, it is definitely not widely publicized.

Even stranger is that Young’s D-day was Dec. 6. Perhaps it was a birthday gift from the Senate.

One other senator has held the title: Senator George Mitchell was deputy president pro tempore in the 100th Congress from 28 January 1987 to 29 November 1988. Walter Mondale would have held the title if he had been reelected to the Senate in 2002, but he lost the election to Norm Coleman.

Under Senate Resolution 17 (95th Cong. 1977), “any Member of the Senate who has held the Office of President of the United States or Vice President of the United States” automatically becomes deputy president pro tempore. Hubert Humphrey held the title from its creation in January 1977 until his death in January 1978.

Under Senate Resolution 90 (100th Cong. 1987), the Senate provided for a deputy president pro tempore during the 100th Congress, in addition to any senator holding the title under the earlier resolution. A subsequent resolution elected Senator George Mitchell to the office, which he held until his election as majority leader in November 1988.

From Mildred Amer, Major Leadership Election Contests in the Senate: A 27-Year Survey (2001). [Warning: This link leads to a .pdf file.]

Would this apply if a former VP was elected to the Senate and is part of a substantial minority? Let’s assume Dan Quayle was elected to a Senate which is 60 Dem 40 Rep. Would Dan Quayle still be Deputy President Pro Tem? Of course, this would assume that Dan Quayle was elected as one of the 40 Republicans.