[LEAPSECS] My FOSDEM slides

[Martin Burnicki]

Tony Finch wrote:
> Martin Burnicki <martin.burnicki at meinberg.de> wrote:
>>
>> I agree, but as I've tried to point out above I think a better project design
>> would have been to use TAI instead of GPS time. PTP works natively with TAI,
>> and you can easily convert between he two scales.
>
> I don't understand this paragraph. The three timescales you mention have
> totally different formats:
>
> TAI: YYYY-MM-DD T HH:MM:SS
> PTP: SSSSSSSSSS
> GPS: WWWW SSSSSS
>
> They have different epochs:
>
> TAI: 1958-01-01 T 00:00:00 Z
> PTP: 1970-01-01 T 00:00:00 Z
> GPS: 1980-01-06 T 00:00:00 Z
>
> So I don't see how it makes sense to argue that PTP is more like TAI than
> GPS time.

In the strict scientific sense I agree.

However, in practice, and for "current" dates, you can convert each of 
them to a number of seconds since the epoch, and for practical purposes 
the difference is just an integral number of seconds.

For example, the IEEE Std 1588-2008 says:

"The PTP epoch is 1 January 1970 00:00:00 TAI, which is 31 December 1969 
23:59:51.999918 UTC"

However, the time stamps used in the PTP packets are just binary numbers 
of seconds, and you have to apply an integral number of seconds to 
convert this to UTC.

Similarly, the comments in the NIST leap second file say:

#	The second column shows the number of seconds that
#	must be added to UTC to compute TAI for any timestamp
#	at or after that epoch.

Even though the number of seconds between TAI and GPS timestamps is 
constant, if you use all the scales TAI/GPS/UTC in an application you 
have to do more different conversions than if you had only TAI and UTC.

Just imagine a PTP grandmaster, controlled by a GPS receiver which 
outputs raw GPS time.

Tony, I'm sure you know all of the above, but I just wrote this to make 
clear what I meant.

Martin
-- 
Martin Burnicki

Senior Software Engineer

MEINBERG Funkuhren GmbH & Co. KG
Email: martin.burnicki at meinberg.de
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