According to this link:
http://linuxtoday.com/news_story.php3?ltsn=1998-12-29-002-10-NW-CY
a machine could have done about 992 days (2.7 years) IF it had not been 
shutdown for UPS maintenance.  That seems about right and is supported 
by this excerpt from the SLUG mailing list:
------------------------------------------
Date: Sun, 11 Nov 2001 22:52:30 EST
Subject: Re: [SLUG] SuSE 7.3 retail
<snip>
....Until the storm last ?month? I had uptimes of 2 years on two of my 
boxes.
Scot Mc Pherson
-------------------------------------------
Starting with the 1.2 kernel I believe there's a jiffy counter that will 
over flow and reset back to zero after so many days (440+??).  Kernels 
prior to 1.2 that were the subject of a legend I once heard.  The story 
  told of the existence of a pre-1.2 box somewhere that has been in 
production continuously since before NT was released.  That would put it 
about 7 years.  Anybody heard that one?
I personally experienced about 5-6 months on a 2.0.x kernel.  It would 
have been 3 more months if I had plugged the machine into the correct 
UPS outlet when I first got a UPS.  The uptime would have been longer 
but I had to shut this machine down and move it for a carpet installation.
Ed.
Bill wrote:
> t seems to me that I have read of Linux boxes with uptimes of years. Does 
> anyone have a link to a web page about this topic?
> 
> Bill
> 
> 
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