Re: 64bit time_t

On Sat, 21 Jun 2008, Chad Giffin wrote:
>
> We currently (normally) store time as the number of elapsed seconds
> since January 1st 1970 GMT.

NTP represents time as an unsigned 32 bit integral number of seconds since
1900-01-01 00:00:00 plus 32 bits of fraction, i.e. the whole timestamp is
64 bit fixed point. This wraps a couple of years before 32 bit Unix time_t.

The TICTOC working group was recently chartered to specify highly accurate
time and frequency distribution. See
http://www.ietf.org/html.charters/tictoc-charter.html

It's not possible to specify a timestamp format that both fits in 64 bits
and has sufficient range and precision to be useful for most purposes.
FreeBSD's bintime format uses standard time_t (eventually 64 bits) plus 64
bits of fractional seconds, i.e. the whole timestamp is 128 bit fixed
point. The high precision avoids aliasing problems when representing
multi-GHz clock frequencies (e.g. CPU clocks).

Tony.
-- 
f.anthony.n.finch  <dot@dotat.at>  http://dotat.at/
HEBRIDES BAILEY: NORTH 5 TO 7, BUT GALE 8 AT FIRST IN HEBRIDES, BACKING WEST 3
OR 4 LATER. ROUGH OR VERY ROUGH, DECREASING MODERATE LATER. SHOWERS. GOOD.
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