* "($Dd,$Dh,$Dm,$Ds) = Delta_DHMS($year1,$month1,$day1,
  $hour1,$min1,$sec1, $year2,$month2,$day2, $hour2,$min2,$sec2);"

  This function returns the difference in days, hours, minutes and
  seconds between the two given dates with times.

  All four return values will be positive if the two dates are in
  chronological order, i.e., if date #1 comes chronologically BEFORE
  date #2, and negative (in all four return values!) if the order of the
  two dates is reversed.

  This is so that the two functions ""Delta_DHMS()"" and
  ""Add_Delta_DHMS()"" (description see further below) are
  complementary, i.e., mutually inverse:

    Add_Delta_DHMS(@date1,@time1, Delta_DHMS(@date1,@time1, @date2,@time2))

  yields ""(@date2,@time2)"" again, whereas

    Add_Delta_DHMS(@date2,@time2,
        map(-$_, Delta_DHMS(@date1,@time1, @date2,@time2)))

  yields ""(@date1,@time1)"", and

    Delta_DHMS(@date1,@time1, Add_Delta_DHMS(@date1,@time1, @delta))

  yields ""@delta"" again.

  The result is zero (in all four return values) if the two dates and
  times are identical.