[LEAPSECS] JD & MJD, UT1 & UTC

[Steve Summit]

zephram wrote:
> It is sometimes useful to split an MJD value into integral and fractional
> parts.  The integral part is the Modified Julian Day Number (MJDN),
> and I call the fractional part the Modified Julian Day Fraction (MJDF).
> When applied to a leapless time scale [...] MJDF is always in the range
> [0, 1).  But when applied to UTC [...] MJDF gives the time within that day
> (even if that's more than 86400 seconds).  The (MJDN, MJDF) tuple is
> unambiguous: during a leap second MJDF >= 1.

And this is eerily similar to the idea of using a struct
timespec with a nonnormalized tv_nsec field.  (Or, for that
matter, the UTC-aware son-of-time_t representation I've been
exploring using the pair (days since 1970, seconds within day).)

> If one wants to display a UTC-based MJD value, it's tempting to adopt
> a digit with value ten, and use that in the first fractional place to
> distinguish a leap second from the following second.

Now that's a cute idea.  (But when you say "It's tempting",
do you really mean "It's tempting, but don't do it"? :-) )