# Re: The real problem with leap seconds

```> We've recently had a question about this on this list which
> wasn't answered clearly.  MJD 27123.5 means 12:00:00 on day
> 27123 if it's not a leap second day, but what does it mean
> on a day with a positive leap second?  12:00:00.5?  I think
> it only works if that level of precision doesn't matter but
> maybe some document somewhere has a convention.```
```
I'm not the expert, but I just read through

http://en.wikipedia.org/wiki/Julian_day

and from what I learned there the answer appears to be that MJD can be
either MJD(UT) or MJD(TT), and leap seconds are not involved.  So MJD
27123.5 means 12:00:00.0000 on day 27123.

MJD(UT) 27123.5 means UT 12:00:00.00000 on day 27123.

MJD(TT) 27123.5 means TT 12:00:00.00000 on day 27123.

UTC is an approximation of UT, perhaps the poorest one in the family
of UT time scales.     If you care about what time it is UT to better
than one second, then UTC is probably not the right time scale for you
to be using (at least not directly).

If a fuzz of +/- 1 second doesn't bother you, then you can pretend
that UTC is UT, and things are easier.

For the time scale experts on this list, did I get that right?

-Tim Shepard
[EMAIL PROTECTED]
```