Re: [slang-users] TIme Routines.

["John E. Davis" <davis@xxxxxxxxxxxxx>]

Ben Duncan <ben@xxxxxxxxxxxxxxxxxx> wrote:
>Anybody know of how to get mktime or has a routine to return
>time values BEFORE the Epoch (12/31/1969).

I have no problem with such values under Linux (debian woody, using
glibc):

  slsh> tm = localtime (-100000000);        
  slsh> print (tm);
  {tm_sec=20, tm_min=13, tm_hour=9, tm_mday=31, tm_mon=9, tm_year=66, tm_wday=1, tm_yday=303, tm_isdst=0}
  slsh> mktime (tm);
  -100000000

What OS are you using?

FWIW, here are some routines that I use to convert Unix time_t values
to and from Julian dates:

% This algorithm came from the calendar FAQ, and appears to have been derived
% from "A Machine Algorithm for Processing Calendar Dates" 
% by Fliegel and Flandern in Communications of the ACM from 1968.
define tms_to_jd (tms)
{
   variable month = tms.tm_mon + 1;
   variable a = (14 - month)/12;
   variable y = (1900+tms.tm_year)+4800-a;
   variable m = month + 12*a - 3;
   variable day = tms.tm_mday;

   variable jd = day + (153*m+2)/5 + y*365 + y/4 - y/100 + y/400 - 32045;
   
   % The julian day begins at noon
   jd -= 0.5;
   return jd + (tms.tm_hour + (tms.tm_min + tms.tm_sec/60.0)/60.0)/24.0;
}

define tms_to_mjd (tms)
{
   return tms_to_jd (tms) - 2400000.5;
}


define unix_to_mjd (t)
{
   return tms_to_mjd (gmtime (t));
}

define mjd_to_unix (mjd)
{
   variable t0 = 0;
   variable t1 = 0x7FFFFFFFU;
   variable mjd0 = unix_to_mjd (t0);
   variable mjd1 = unix_to_mjd (t1);

   if ((mjd0 > mjd) or (mjd > mjd1))
     verror ("%S: date cannot be represented as a unix time_t", _function_name);

   forever
     {
	variable t = t0 + (t1-t0)/2;
	variable mjd2 = unix_to_mjd (t);

	if (mjd2 <= mjd)
	  {
	     if (mjd2 == mjd)
	       return t;

	     mjd0 = mjd2;
	     t0 = t;
	     continue;
	  }
	mjd1 = mjd2;
	t1 = t;
     }
}

define mjd_to_year (mjd)
{
   variable t = mjd_to_unix (mjd);
   variable tm = gmtime (t);
   variable y = 1900.0 + tm.tm_year;
   variable d = tm.tm_yday + (tm.tm_hour + (tm.tm_min+tm.tm_sec/60.0)/60.0)/24.0;
   y += d/365.0;
   return y;
}


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