# Take a decoded quadword that's got a VMS time in it and convert it t
+o epoch
# seconds. VMS dates are a 63 bit count of units since Nov 17, 1858. (
+Units
# are 100ns long), while unix dates are in seconds since Jan 1, 1970. 
+FWIW,
# there are 3,506,716,800 seconds between VMS' base date and Unix/Perl
+'s
# base date.
#
# First parameter is a scalar with the integer value of the low 32
# bits, second parameter is the integer value with the high 32
# bits. You'd call it like:
#
# $time = quad_to_epoch(423828480, 9926120);
#
sub quad_to_epoch() {
  my ($low_long, $high_long, $VMS_based_time, $Unix_based_time);
  $low_long = int shift(@_);
  $high_long = int shift(@_);
  # VMS' time has lots of extra bits we don't care about. There are 10
+^7
  # ticks per second, so we can toss all the sub-second bits. 10^7 ==
  # 1000000 == 0x989680 == 100110001001011010000000. The low seven bit
+s
  # are all zero, so we can shift everything to the right 7 bits then
  # divide by 78125.
  #
  # We play these games because of precision. A VMS date has 63 bits, 
+but
  # an integer only has 32. OTOH, a double has 53 bits. Which means if
+ we
  # shift the individual pieces by 7, we only have 3 bits we could los
+e,
  # and only if one of the top three bits of the VMS date are ones. Th
+is
  # won't happen until well after the year 3000, so I expect we're saf
+e
  # for a while.
  #
  # First we shift the low longword right 7 bits and mask off the top 
+7
  # (in case we sign extended)
  $low_long = ($low_long >> 7) & 0x01ffffff;
  # Find the low seven bits of our high longword and tack them on to t
+he
  # top of our low word. As a result of these games, $low_long gets
  # morphed from an IV to an NV. The shift's 24 with a multiply by 2
  # (instead of a shift by 25) to avoid shifting into the sign bit
  $low_long = $low_long + ((($high_long & 127) << 24) * 2);
  # Shift the high word down by seven bits (the ones we saved). No
  # masking's needed, as we know we can't sign extend. (VMS absolute
  # times are guaranteed to have the high bit clear)
  $high_long >>= 7;
  # Put 'em together
  $VMS_based_time = int((($high_long * 4294967296) + $low_long) / 7812
+5);
  $Unix_based_time = $VMS_based_time - 3_506_716_800;
  return $Unix_based_time;
}
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