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

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 https://en.wikipedia.org/wiki/International_yard_and_pound).