[ntp:questions] Re: NTP, Unix and the 34-year itch (better read this)
David L. Mills
mills at udel.edu
Thu Jan 22 20:29:20 UTC 2004
My computers won't boot if the CMOS battery runs down. Even if it did
and the TOY reset, on the way back after battery replacement the TOY
would start counting from New Year and in the usual case have to be
I only know of two manufacturers that have ported NTPv4 to an embedded
operating system and they probably have changed a bunch of things. I
don't think they would be thrilled at re-porting should ntpd be "fixed"
in some form. Much easier from them to change the firmware. In Linksys
sources one line would do it.
Terje Mathisen wrote:
> Hal Murray wrote:
> >>Almost all computers of today have some means, such as a time-of-year
> >>(TOY) chip, to set the system clock at power up or reboot surely within
> >>within 34 years, but some embedded systems do not. For embedded systems
> >>without a TOY chip and running an embedded Unix kernel, the initial time
> >>is usually the Unix base epoch 1 January 1970. Readers will quickly
> >>realize the time since then now in 2004 exceeds the 34-year limit. These
> >>systems have a problem unless something is done.
> > Another case to consider is the battery running down. That happens
> > after a while if the box is not plugged in.
> > Another way to get a good starting epoch would be to use the
> > build time of the kernel.
> Even better, and easier to get for an ntpd deamon:
> The build time of the ntpd deamon itself: This will then be good for the
> next 34 years, and any system which can run without recompilation for 34
> years deserves a _lot_ of respect.
> In fact, I suspect such a system will be well-engineered enough to have
> a working TOY clock.
> Even if the TOY clock breaks (battery stops working after ~5-10 years),
> simply demanding that the initial time is >= the build time would suffice.
> Alternatively, use the file system level timestamp of the ntpd deamon:
> If you're really paranoid/scared, you could run 'touch ntpd' (or a
> companion date flag file) as part of the normal shutdown sequence, this
> would work for all systems with less than 34 years between reboots.
> However, since the 34-year limit is for ntp time differences, a 68 or
> even 136 year range should work for ntp absolute times, i.e. ntpdate or
> ntpd -g initial time setting.
> - <Terje.Mathisen at hda.hydro.com>
> "almost all programming can be viewed as an exercise in caching"
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