[LEAPSECS] leapseconds, converting between GPS time (week, second) and UTC

[jimlux]

On 1/16/19 12:31 AM, Poul-Henning Kamp wrote:
> --------
> In message <20190115205243.GB25854 at ucolick.org>, Steve Allen writes:
>> On Tue 2019-01-15T12:34:11-0800 Gary E. Miller hath writ:
>>> Yes, and no.  time_t is just seconds since an epoch.  Which epoch
>>> is not well defined.  The epoch may well be anything.  See "man difftime".
>>
>> That evokes a challenge for all time nuts that I can make based on
>> reading Bulletin Horaire.
>>
>> What is the epoch that was used for TAI?
> 
> Isn't that the same one Loran-C used ?  1958-01-01 00:00:00 GMT ?
> 
> 
You might be thinking of CCSDS Unsegmented Time - in the header of the 
time message you specify what the epoch is - It's either the 1958 
(beginning of "space age") or "user defined"

Similar for CCSDS Day Segmented Code which is 16 or 24 bits of days 
after epoch, milliseconds, and (optional) submilliseconds

They also clone the ISO UTC called ASCII Calendar Segmented Time Code  - 
YYYY-MM-DDThh:mm:ss.d....dZ as well as DOY versions, etc.

They manage the leapsecond with a 58 or 61 second minute. It's also 
clear that CCSDS doesn't like non-continuous, non-monotonic time scales:

"
f) Standardization on the use of these time code formats for purposes 
OTHER than identifying an instant of calendar or time in UTC (e.g., 
unconventional use as a counter or tool for measuring arbitrary 
intervals) is not recommended. It is felt such a specialized application 
can best be viewed not as a time code format but rather as an 
engineering measurement format. Any such application of these time code 
formats is considered beyond the scope of this Recommended Standard.
"





https://public.ccsds.org/Pubs/301x0b4e1.pdf

"The CCSDS-Recommended epoch is that of 1958 January 1 (TAI) and the 
recommended time unit is the second, using TAI as reference time scale, 
for use as a level 1 time code. This time code is not UTC-based and 
leap-second corrections do not apply."
Bit1-3 =Time code identification
  001 — 1958 January 1 epoch (Level 1 Time Code)
  010 — Agency-defined epoch (Level 2 Time Code)


Level 1 in CCSDS means "complete and unambiguous interpretation", Level 
2 means "you need some other information"