# [tz] What's "right"?

Michael H Deckers michael.h.deckers at googlemail.com
Tue Nov 17 14:02:51 UTC 2020

```   On 2020-11-16 21:11, Paul Gilmartin wrote:

> A "negative leap second" is problematic.  It would be a TAI second
> with no matching UTC second.

No.

Let us assume that a negative leap second will be "dropped"
in UTC at 2028-01-01, so that
for UTC directly before       2028-01-01, one has TAI - UTC =
37 s
for UTC on and directly after 2028-01-01, one has TAI - UTC =
36 s.
Then there are successive timestamps 2027-12-31T23:59:57
2027-12-31T23:59:58
2028-01-01T00:00:00
in UTC when TAI is one of 2028-01-01 + {34, 35, 36} s.
In ITU lingo, the negative leap second is 2027-12-31T23:59:59.
There is no "corresponding second of" TAI.

To make matters even more confusing, consider that
TAI "counts seconds" including positive leap seconds
but excluding negative leap seconds, and
UTC "counts seconds" excluding positive leap seconds
but including negative leap seconds.

Michael Deckers.

```