! This file: http://ftp.aset.psu.edu/pub/ger/fortran/hdk/estdst,f90
!
!====Compute date for year, YYYY, for changing clocks to DST and EST.
! Since 1966, most of the United States has observed Daylight
! Saving Time on the first Sunday of April until the last Sunday
! of October. Beginning in 2007, most of the U.S. will begin Daylight
! Saving Time on the second Sunday in March and revert back to
! Standard Time on the first Sunday in November. In the U.S.
! each time zone switches to and from DST at a different time.
! hdk - 30 November 2006

      CHARACTER(60) LINE
      INTEGER YYYY,MM,DD,I,WEEKDAY, EOF
      INTEGER, DIMENSION(8) :: VAL
!------WEEKDAY Gives the weekday number 0=SUNDAY, 1=MONDAY,
!      ... 6=SATURDAY.
      WEEKDAY(YYYY,MM)=MOD((13*(MM+10-(MM+10)/13*12)-1)/5+DD+77  &
         +5*(YYYY+(MM-14)/12-(YYYY+(MM-14)/12)/100*100)/4        &
         + (YYYY+(MM-14)/12)/400-(YYYY+(MM-14)/12)/100*2,7)

 1    WRITE(*,*) 'Please enter a four-digit year (YYYY) > 1965;'
      WRITE(*,*) '       or press Enter for current year:'
      READ(*,'(A)',IOSTAT=EOF) LINE
      IF (EOF.EQ.0 .AND. LINE .NE.'') THEN
        READ(LINE,*) YYYY
      ELSE
!  If no year entered, then compute date this year.
        CALL DATE_AND_TIME(VALUES=VAL)
        YYYY = VAL(1)
      ENDIF

! Compute Eastern Standard and Daylight Savings Time Change.
      IF (YYYY .LT. 1966) GOTO 1
      IF (YYYY .LT. 2007) THEN

       MM=10    ! EST on October 2006 and before.
       DO I=31,26,-1   ! Look backwards for the last Sunday in October.
         DD=I
         IF (WEEKDAY(YYYY,MM) .EQ. 0) THEN
          WRITE(*,*) 'Turn Clocks back to EST on: ',I,' October ',YYYY
          EXIT
         ENDIF
       END DO

       MM=4    ! DST on April 2006 and before.
       DO I=1,7   ! Look for first Sunday in April.
         DD=I
          IF (WEEKDAY(YYYY,MM) .EQ. 0) THEN
           WRITE(*,*) 'Turn Clocks forward to DST on: ',I,' April', YYYY
          EXIT
          ENDIF
       END DO

      ELSE
       MM=11    ! EST on November 2007 and after.
       DO I=1,7 ! Look for first Sunday in November.
         DD=I
         IF (WEEKDAY(YYYY,MM) .EQ. 0) THEN
          WRITE(*,*) 'Turn Clocks back to EST on: ',I,' November ',YYYY
          EXIT
         ENDIF
       END DO

       MM=3    ! DST on March 2007 and after.
       DO I=14,1,-1 ! Look for second Sunday in March
         DD=I
         IF (WEEKDAY(YYYY,MM) .EQ. 0) THEN
          WRITE(*,*) 'Turn Clocks forward to DST on: ',I,' March ',YYYY
          EXIT
         ENDIF
       END DO

      ENDIF

      STOP
      END