Re: Ada.Calendar and NTP (and Unix Epoch)

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Tue, 24 Jul 2012 01:34:03 -0700 (PDT), erlo.haugen@gmail.com wrote:

>> Political time is one regulated by politicians, localized, daylight saving
>> etc. Political time is unusable for the purpose of time stamping and clock
>> synchronization, but of course indispensable for the UI.
>> 
>> Ada.Calendar implicitly represents such a time. NTP, I only guess, because
>> we are using other mechanisms of time distribution and synchronization, is
>> not a political time. Is it UTC?
>> 
>> Which is why I second to Adam asking what are going to achieve.
> 
> I see. NTP is just the number of seconds

Any time is just a number of seconds added to some epoch...

> The purpose of this exercise is to compare two timestamps, one base on
> Unix epoch and one based on NTP epoch.

Where is a problem then?

   E1 + T1 vs. E2 + T2

<=>

   T1 vs. (E2 - E1) + T2

Where E2 - E1 is the time offset between the epochs.

This presumes that they represent *same* time though maybe using different
epochs. You might need to read the UNIX and NTP documentation to learn what
exactly times they do represent. That would give you E2 - E1. To put it
simple: same time = synchronized clocks.

Note that if any of two times is political, E2 - E1 does not exist as it
becomes a function of time. This where all starts really complicated and is
the reason why one should never ever mess up with the political time for
the purpose of time stamping. And this is just a relatively simple case
when clocks are synchronized but one of them is periodically skewed.

When clocks are not synchronized, time stamps are fundamentally
incomparable without the history of synchronization, which, well, means
that they were rather synchronized. (:-))

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de