Re: [Bug-tar] tar writes invalid mtime for times before the epoch

[Paul Eggert]

On 11/20/2017 02:51 AM, Joerg Schilling wrote:
> > That's a shortcoming in tar's documentation. Tar uses the GNU format by default,
> > which has a base-256 extension that supports negative timestamps. If you want
> > GNU tar to refuse to use this GNU extension, please use '-H ustar'.
> Well, base-256 is not even special to GNU tar. It has been introduced by star
> before and was later adopted by GNU tar.

As I vaguely recall, this extension was designed by both of us in 
collaboration, and superseded an earlier base-256 format that GNU tar 
still supports but does not document. I'm too lazy to consult the email 
archives to check my memory; it's not a big deal either way.

> The background is that base-256 allows up to 95 bits + sign bit and this is
> sufficient for all possible storage as long as you cannot manage to store part
> of the data in a parallel universe.
No parallel universe should be needed to exhaust the format's limits. In 
2001 Seth Lloyd estimated that the observable universe, if treated as a 
computer, would contain about 10**120 bits of information if quantum 
gravity were taken into account. This would require 396 bits to address, 
assuming byte-addressible storage. Admittedly Lloyd's estimate is very 
rough and could well need updating in the light of more-recent physical 
discoveries. Still, 95 bits does not seem to be nearly enough. And even 
if 396 bits is right just now, eventually it'll be too small as the 
number of bits in the universe is growing.

Lloyd S. Computational capacity of the Universe. Phys Rev Lett. 
2002;88(23):237901. http://dx.doi.org/10.1103/PhysRevLett.88.237901