I can see this is an ordinal in the standard definition... in Python, an ordinal (more specifically the ".toordinal()" or .fromordinal() ) is the number of days that have occurred since Jan 01, 01. If that's the case, there may be a method.
"Dates like 733828.0 are Rata Die dates, counted from January 1, 1 A.D. (and decimal fraction of days). They may be UTC or by your timezone."
I attempted something like
select cast(datediff(day,'0000-12-31',cast(getutcdate() as datetime)) as bigint) as ordinaldays
It brings back an error but displays the intent - probably due to the issue of how to convert time when different countries all used different methods and dates to convert to Gregorian... Examples: Spain 1500's, England and America, 1700's, Ireland 1900's... It is easier to do the math on a conversion from 1753, but it can be very wrong in some places because it doesn't present a real solution. Like integer math on 1970 for seconds, there can be problems with the length of a second, or, another eample, with conversions of the 365 year day which is actually more like a fraction = 365.24 roughly, which leads to the 400 year cycle (of 1/1/2000 known as Y2K) and the 4 year cycle (leap year) and so on and so forth.
There is a neat little feature in Python that counts days from an imaginary date of zero that occurs just before the Gregorian calendar begins. It works for timestamping days which can be efficient when using it as an index.