Conference rusure::math

    Convert the dates to Julian date format and subtract the smaller from
    the larger.
    
    The following routines can be used to convert between Julian and
    standard date format. They assume 32-bit longs and would need to
    be adapted if you are bulding 16-bit PC applications.
    
      John
    
unsigned long tojul(unsigned long x)  /* YYMMDD --> Julian date */
{
  float day, month, year, igreg, julday, ja, jy, jm;
  igreg = (15 + 31 * (10 + 12 * 1582));	/* Date of adoption of Greg. Cal. */
  year  = 1900 + (int)(x / 10000);
  if (year < 0.0) year++;
  month = (x/100) - 100 * (x/10000);
  day   = x % 100;
  if (month > 2.0) {jy = year;     jm = month + 1.0;}
  else             {jy = year - 1; jm = month + 13.;}

  julday  = ((unsigned long)(365.250 * jy));
  julday += ((unsigned long)(30.6001 * jm));
  julday += day + 1720995;

  if ((day + 31 * (month + 12 * year)) >= igreg)
  {
    ja = ((unsigned long)(.01 * jy));
    julday += (2.0 - ja);
    julday += (.25 * ja);
  }
  return ((unsigned long)(julday));
}

unsigned long toyymmdd(unsigned long x)	  /* Julian date --> YYMMDD */
{
  long igreg, jalpha, ja, jb, jc, jd, je, id, mm, yy, q;

  igreg = 2299161;
  if (x >= igreg)
  {
    jalpha = (((x - 1867216) - .25) / 36524.25);
    q = .25 * jalpha;
    ja = x + 1 + jalpha - q;
  }
  else ja = x;

  jb = ja + 1524;
  jc = 6680 + ((jb - 2439870) - 122.1) / 365.25;
  q  = .25 * jc;
  jd = 365 * jc + q;
  je = (jb - jd) / 30.6001;
  q  = 30.6001 * je;
  id = jb - jd - q;
  mm = je - 1;
  if (mm > 12) mm = mm - 12;
  yy = jc -4715;
  if (mm > 2) yy--;
  if (yy <= 0) yy--;
  yy = yy - 1900;
  return ((unsigned long)((yy * 10000) + (mm * 100) + id));
}

    re .0
    
    If the dates are 17-NOV-1858 or later
    
    Other approaches are:
    
    Convert both dates to a day number using LIB$DAY and then subtract the
    results.
    
    *or*
    
    If the number of days is <= 9999 then convert both dates to VMS
    absolute date/time format with SYS$BINTIM, then subtract with
    LIB$SUB_TIMES, then convert back to human readable format with
    SYS$ASCTIM.

Thanks for replies, I will try it out this evening.

Bianca.

$! DELTA_DAYS.COM 	J. FINKA    Nov 1990
$! assume p1 <= p2 and delta < 9995
$ p1 = f$cvtime(p1,"absolute","date")
$ p2 = f$cvtime(p2,"absolute","date")
$ y1 = f$integer(f$cvtime(p1,,"year"))
$ y2 = f$integer(f$cvtime(p2,,"year"))
$ m1 = f$integer(f$cvtime(p1,,"month"))
$ m2 = f$integer(f$cvtime(p2,,"month"))
$ d1 = f$integer(f$cvtime(p1,,"day"))
$ d2 = f$integer(f$cvtime(p2,,"day"))
$ y = (36525 * (y2 - y1) + 3044 * (m2 - m1)) / 100 + d2 - d1
$ i = 0
$ l: x = y + i
$ i = - i
$ if i .ge. 0 then i = i + 1
$ q = f$cvtime(p1 + "+" + f$string(x) + "-","absolute","date")
$ if q .nes. p2 then goto l
$ write sys$output "Delta days : ", x

Try this right from character cell notes:

    Notes> spawn @hannah::igloo$:[osman.comfiles]subdat 1-jul-1994 1-apr-1993

And of course this one(!):

    Notes> spawn @hannah::igloo$:[osman.comfiles]subdat 11-jul-1994 1-jan-2000




/Eric

    re. .-1
    
    Hi Eric,
    
      why do I get the same answer for two different dates?
    
    i.e., 
    
    spawn @hannah::igloo$:[osman.comfiles]subdat 20-Nov-1957 27-jul-1994
    spawn @hannah::igloo$:[osman.comfiles]subdat 20-Nov-1963 27-jul-1994
    
    both give: 
    9999 days, 23:59:59.99
    
    /k

        The VMS delta time format does not go to 10,000 days or
        beyond.  It is limited to nnnn hh:mm:ss.cc, the maximum
        being 9999 days 23 hours 59 minutes 59.99 seconds.
        
        Reply .2 mentions the restriction.
        
        Dan