[tz] the final second before standard time daylight savings time transitions

[Paul Eggert]

Inman, Mark H CIV USARMY HQDA (US) wrote:
> Is there any official documentation on the time zone for March 13 2016 in
> New York City, when the clock says 1:59:59.99991000 ?

I don't know about "official", but in practice digital clocks jump from 
01:59:59.99999...9 EST to 03:00:00 EDT. You can use as many 9s as you want, as 
long as it's a finite number of 9s. This is baked into the POSIX API, e.g., the 
nanoseconds component of struct timespec has nine 9s.

This reminds me of the confusion at midnight. Does midnight belong to the 
previous day, to the next day, to both, or to neither? Traditionally, it was 
ambiguous. Nowadays with digital clocks midnight is invariably 00:00 at the 
start of the next day.