Re: [O] dates before 1970

[Nick Dokos]

Carsten Dominik <address@hidden> wrote:

> 
> On Mar 11, 2011, at 5:30 PM, Nick Dokos wrote:
> 
> > Carsten Dominik <address@hidden> wrote:
> > 
> > 
> >> THis is exactly the point, that it depends on how Emacs was compiled, and 
> >> what kind of integer is used in the date representation.  Signed or 
> >> unsigend, 32 or 64 bits (I think).
> >> 
> >> For example, Bastien can represent dates before 1970. I cannot.
> >> I can represent dates after 2038, Bastien cannot.
> >> 
> >> The work-around is to use diary sexps for dates before 1970, that seems to 
> >> be safe.
> >> And then hope that by 2038, all computers will use 64 bit integers....
> >> 
> > 
> > But it's even more than that, no? Emacs's time implementation
> > (current-time, encode/decode etc) would have to change. In fact, this
> > might be the most significant limitation right now: the values they pass
> > around are (hi16 lo16 ms) so they assume that time values are 32 bits,
> > no matter what the underlying implementation says. I use 64-bit Linux on
> > an x86-64 laptop and my time_t is 64 bits (but I don't know if it's
> > signed or unsigned). Time for some experimentation I guess...
> 
> I believe that on your system you might get
> 
> (hi48 lo16 ms)
> 
> You can test if it is signed by trying a date before 1970
> 
>    (encode-time 0 0 0 1 1 1960)
> 

Ah, OK - thanks! I looked in current-time, saw the 0xffff mask and I
thought that the extra bits are truncated, but apparently not: I need to
go back and look at the C rules again.

I get

(encode-time 0 0 0 1 1 1960)
(-4816 20176)

(decode-time '(-4816 20176))
(0 0 0 1 1 1960 5 nil -18000)

so it is indeed signed.

I just pulled latest emacs and apparently Paul Eggert has been active in
making this code more robust, presumably after your conversations on the
emacs list.

Thanks,
Nick