That depends on whether or not you define "day" as mean solar day or as 24 hours; either way you need some sort of ultimate reference in order actually define a meaningful unit. A more stable definition is as 60 SI seconds, since the second is defined relative to a very precise physical reference (level transitions in cesium).

It is for a similar reason that the pound and yard were redefined in the 1960's as an exact multiple of the kilogram and meter, so as to be able to benefit from the exactness of physical definitions for those units (https://en.wikipedia.org/wiki/International_yard_and_pound).

And as shown above OED deals so exceptionally well with all concepts... down to the second. Where it sends you to SI to get the proper definition. Can you show me what OED says for ultrapure?

How about secret and top secret? Ultraspherical and hypergeometric? And don't forget the cleanroom.

Try to put that "wit" into understanding that in highly specialized fields the terminology might get different meanings and none of it is wrong.

What if there is a leap second on that day?