anchor.diff:  PURE ELEMENTAL SUBROUTINE addDays(self,d)
   !======================================================================>
   !
  -! datetime-bound procedure. It adds an integer number of days to self. 
  +! datetime-bound procedure. Adds an integer number of days to self. 
   ! Called by datetime addition (+) and subtraction (-) operators.
   !
   !======================================================================>
  
positive.code: 
    INTEGER :: year
  
    date2num = 0
    DO year = 1,d%year-1
      date2num = date2num+daysInYear(year)
  
negative.code:   REAL(KIND=real_dp)            :: days,totseconds
  
    INTEGER :: year,month,day,hour,minute,second,millisecond
  
    ! TODO: num must be positive. Implement checks.
  
    days = num
  
  