!********************************************
!* Compute number of days between two dates *
!* ---------------------------------------- *
!* Ref.: "Mathématiques par l'informatique  *
!*        individuelle (1) par H. Lehning   *
!*        et D. Jakubowicz, Masson, Paris,  *
!*        1982" [BIBLI 06].                 *
!*                                          *
!*        F90 version by J-P Moreau, Paris. *
!*                (www.jpmoreau.fr)         *
!********************************************

!*****************************************************************************
! Note: The two dates must be between 01/01/1901
!       and 12/31/2099 to avoid treatment of special
!       "secular" years, such as 1800, 1900, 2100...
!       (2000, a multiple of 400, is a normal year!)!
!
! The first date is M, D, Y
! N, number of days between first date and fictitious
! starting date 01/00/1901, is given by formula:
!
!       N = INT(365,25*(Y-1901)+A(M)+D)
!
! where A(M) is given by table below:
!
! M    1  2  3     4      5      6      7      8      9     10     11     12
! A(M) 0 31 59,25 90,25 120,25 151,25 181,25 212,25 243,25 273,25 304,25 334,25
!
!       First N is saved in R.
!
! The second date is again M, D, Y
! N is again computed with same formula.
!
! Final result is N - R.
!*******************************************************************************
PROGRAM NDAYS


  REAL*4  A(12)
  REAL*4  N                  ! number of days
  INTEGER R                  ! number of days 

  A(1) = 0; A(2) = 31; A(3) = 59.25; A(4) = 90.25; A(5) = 120.25; A(6) = 151.25
  A(7) = 181.25; A(8) = 212.25; A(9) = 243.25; A(10) = 273.25; A(11) = 304.25
  A(12) = 334.25

  print *,''
  print *,' Compute number of days between two dates'
  print *,'     (from 01/01/1901 to 12/31/2099)'
  print *,''
  print *,' Enter first date (M D Y): '
  call Calcul(A,N)
  R = N
  print *,' Enter second date (M D Y): '
  call Calcul(A,N)
  R = N - R
  print *,''
  print *,' Number of days: ',R
  print *,''; print *,''
  stop
end

Subroutine Calcul(A,N)
  integer M,D,Y   !month,day,year
  REAL*4 N,A(12)
  read *, M, D, Y
  N = 365.25 * (Y - 1901) + A(M) + D
  N = int(N)
  return
end

! end of file ndays.f90