Prior to 1972-01-01 UTC days were not an integer number of SI seconds in length. E.g., in the four years immediately prior to this date:
TAI-UTC = 4.213 170 0 + (MJD - 39 126) � 0.002 592s from: http://hpiers.obspm.fr/eop-pc/earthor/utc/TAI-UTC_tab.html This means that a UTC day was roughly two and a half milliseconds longer than 86 400 SI seconds. Here's the question: was a UTC second the same as an SI second so that days were a non-integral number of UTC seconds or was the UTC second slightly longer than the SI second? Putting the same question another way: was the MJD date considered to be an integer or a real day number? I.e., did TAI-UTC vary during the course of a single UTC day? This question is mostly just for curiousity but it is slightly relevant when people define time scales supposedly based on an epoch such as 1970-01-01T00:00:00Z. More on this later. Ed.
