subroutine dcdate_normalize(day, sec, day_seconds)
    !
    !=== 日と秒の正規化
    !
    ! このサブルーチンは内部向けなので dc_date モジュール外では
    ! 極力使用しないでください.
    !
    ! 日付 *day* と秒数 *sec* の正規化を行います. *sec* が *day_seconds*
    ! (省略される場合は dc_date_types#day_seconds) を超える場合, *day*
    ! に繰上げを行います.
    ! また, *sec* と *day* の符号が逆の場合, 同符号になるよう
    ! 設定します.
    !
    use dc_date_types, only: day_seconds_default => day_seconds
    implicit none
    integer, intent(inout):: day
    real(DP), intent(inout):: sec
    real(DP), intent(in), optional:: day_seconds
    integer:: sgn
    real(DP):: day_sec
  continue
    if (present(day_seconds)) then
      day_sec = day_seconds
    else
      day_sec = day_seconds_default
    end if
    if (abs(sec) > day_sec) then
      day = day + int(sec / day_sec)
      sec = modulo(sec, day_sec)
    end if
    if ((sec > 0.0 .and. day < 0) .or. (sec < 0.0 .and. day > 0)) then
      sgn = sign(day, 1)
      day = day - sgn
      sec = sec + sgn * day_sec
    endif
  end subroutine dcdate_normalize

  character(TOKEN) function dcdate_parse_unit(str) result(unit)
    !
    ! このサブルーチンは内部向けなので dc_date モジュール外では
    ! 極力使用しないでください.
    !
    ! 引数 *str* に与えられた文字列を解釈し, 日時の単位を
    ! 返します. それぞれ以下の文字列が日時の単位として解釈されます.
    ! 大文字と小文字は区別されません.
    ! 返る文字列は以下の文字型の配列の先頭の文字列です.
    ! (例: *str* に 'hrs.' が与えられる場合, dc_date_types#UNIT_HOUR
    ! 配列の先頭の文字列 UNIT_HOUR(1) が返ります.)
    !
    ! 年 :: dc_date_types#UNIT_YEAR
    ! 月 :: dc_date_types#UNIT_MONTH
    ! 日 :: dc_date_types#UNIT_DAY
    ! 時 :: dc_date_types#UNIT_HOUR
    ! 分 :: dc_date_types#UNIT_MIN
    ! 秒 :: dc_date_types#UNIT_SEC
    !
    ! これらに該当しない文字列を *str* に与えた場合, 空文字が返ります.
    !
    use dc_types, only: TOKEN
    use dc_date_types, only: UNIT_YEAR, UNIT_MONTH, UNIT_DAY, UNIT_HOUR, UNIT_MIN, UNIT_SEC
    use dc_string, only: StriEq
    implicit none
    character(*), intent(in):: str
    integer :: unit_str_size, i
  continue
    unit = adjustl(str)
    unit_str_size = size(UNIT_SEC)
    do i = 1, unit_str_size
      if (StriEq(trim(unit), trim(UNIT_SEC(i)))) then
        unit = UNIT_SEC(1)
        return
      end if
    end do

    unit_str_size = size(UNIT_MIN)
    do i = 1, unit_str_size
      if (StriEq(trim(unit), trim(UNIT_MIN(i)))) then
        unit = UNIT_MIN(1)
        return
      end if
    end do

    unit_str_size = size(UNIT_HOUR)
    do i = 1, unit_str_size
      if (StriEq(trim(unit), trim(UNIT_HOUR(i)))) then
        unit = UNIT_HOUR(1)
        return
      end if
    end do

    unit_str_size = size(UNIT_DAY)
    do i = 1, unit_str_size
      if (StriEq(trim(unit), trim(UNIT_DAY(i)))) then
        unit = UNIT_DAY(1)
        return
      end if
    end do

    unit_str_size = size(UNIT_MONTH)
    do i = 1, unit_str_size
      if (StriEq(trim(unit), trim(UNIT_MONTH(i)))) then
        unit = UNIT_MONTH(1)
        return
      end if
    end do

    unit_str_size = size(UNIT_YEAR)
    do i = 1, unit_str_size
      if (StriEq(trim(unit), trim(UNIT_YEAR(i)))) then
        unit = UNIT_YEAR(1)
        return
      end if
    end do

    unit = ''

  end function dcdate_parse_unit

  type(DC_DIFFTIME) function dcdate_add_ff(diff1, diff2) result(result)
    implicit none
    type(DC_DIFFTIME), intent(in):: diff1, diff2
  continue
    result % mon = diff1 % mon + diff2 % mon
    result % day = diff1 % day + diff2 % day
    result % sec = diff1 % sec + diff2 % sec
    result % day_seconds = diff1 % day_seconds
    call dcdate_normalize(result % day, result % sec, result % day_seconds)
  end function dcdate_add_ff

  type(DC_DATETIME) function dcdate_add_ft(diff, time) result(result)
    !
    ! 2 つの日時 (DC_DATETIME 型) もしくは
    ! 日時差 (DC_DIFFTIME 型)の加算を行います.
    !
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    type(DC_DATETIME), intent(in):: time
    integer:: time_year, time_mon, time_day, time_caltype
    real(DP):: time_sec
    character(6):: time_zone
  continue
    call Eval(time, year = time_year, mon = time_mon, day = time_day, sec = time_sec, caltype = time_caltype, zone = time_zone)
    call Create(result, year=time_year, mon = time_mon + diff % mon, day = time_day + diff % day, sec = time_sec + diff % sec, caltype = time_caltype, zone = time_zone)
  end function dcdate_add_ft

  type(DC_DATETIME) function dcdate_add_tf(time, diff) result(result)
    use dc_types, only: DP
    implicit none
    type(DC_DATETIME), intent(in):: time
    type(DC_DIFFTIME), intent(in):: diff
  continue
    result = dcdate_add_ft(diff, time)
  end function dcdate_add_tf

  type(DC_DIFFTIME) function dcdate_div_fd(diff, denominator) result(result)
    !
    ! ※ 注意 : 月差を除算すると近似的結果になるおそれがあります
    use dc_date_types, only: CYCLIC_MDAYS
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    real(DP), intent(in):: denominator
    real(DP):: month, day
  continue
    month = diff % mon / denominator
    result % mon = int(month)
    day = diff % day / denominator + int(CYCLIC_MDAYS * (month - result % mon))
    result % day = int(day)
    result % sec = diff % sec / denominator + (day - result % day) * diff % day_seconds
    result % day_seconds = diff % day_seconds
    call dcdate_normalize(result % day, result % sec, result % day_seconds)
  end function dcdate_div_fd

  real(DP) function dcdate_div_ff(diff1, diff2) result(result)
    !
    ! ※ 注意 : 月差と日時の混在する除算は近似的結果になるおそれがあります
    use dc_date_types, only: CYCLIC_MDAYS
    implicit none
    type(DC_DIFFTIME), intent(in):: diff1, diff2
  continue
    ! ゼロ割対応コードが必要か?
    result = (diff1 % day_seconds * (CYCLIC_MDAYS * diff1 % mon + diff1 % day) + diff1 % sec) / (diff2 % day_seconds * (CYCLIC_MDAYS * diff2 % mon + diff2 % day) + diff2 % sec)
  end function dcdate_div_ff

  type(DC_DIFFTIME) function dcdate_div_fi(diff, denominator) result(result)
    !
    ! 日時差 *diff* を *denominator* で除算した結果を返します.
    !
    ! ※ 注意 : 月差を除算すると近似的結果になるおそれがあります
    use dc_date_types, only: CYCLIC_MDAYS
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    integer, intent(in):: denominator
  continue
    result % mon = diff % mon / denominator
    ! 月からの近似的繰り下がりは日単位でしか行わない
    result % day = diff % day / denominator + int((CYCLIC_MDAYS * mod(diff % mon, denominator)) / denominator)
    result % sec = diff % sec / denominator + (diff % day_seconds * mod(diff % day, denominator)) / denominator
  end function dcdate_div_fi

  type(DC_DIFFTIME) function dcdate_div_fr(diff, denominator) result(result)
    !
    ! ※ 注意 : 月差を除算すると近似的結果になるおそれがあります
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    real, intent(in):: denominator
  continue
    result = dcdate_div_fd(diff, real(denominator, DP))
  end function dcdate_div_fr

  logical function dcdate_eq_df(d, diff) result(result)
    !
    ! 引数 *diff* の日時差が *d* と等しいかどうかを比較します. *diff*
    ! を秒数に換算した値と *d* とが等しい場合, .true. が返ります.
    !
    use dc_types, only: DP
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    real(DP), intent(in):: d
  continue
    if (real(EvalSec(diff)) == d) then
      result = .true.
    else
      result = .false.
    end if
  end function dcdate_eq_df

  logical function dcdate_eq_fd(diff, d) result(result)
    use dc_types, only: DP
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    real(DP), intent(in):: d
  continue
    result = dcdate_eq_df(d, diff)
  end function dcdate_eq_fd

  logical function dcdate_eq_ff(diff1, diff2) result(result)
    !
    ! 2 つの引数の日時差を比較します.
    ! 1 つ目の引数に格納される日時差が 2 つ目の引数に格納される日時差
    ! と同じ場合, .true. が返ります.
    !
    implicit none
    type(DC_DIFFTIME), intent(in):: diff1, diff2
  continue
    if (EvalSec(diff1) == EvalSec(diff2)) then
      result = .true.
    else
      result = .false.
    end if
  end function dcdate_eq_ff

  logical function dcdate_eq_fi(diff, i) result(result)
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    integer, intent(in):: i
  continue
    result = dcdate_eq_if(i, diff)
  end function dcdate_eq_fi

  logical function dcdate_eq_fr(diff, r) result(result)
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    real, intent(in):: r
  continue
    result = dcdate_eq_rf(r, diff)
  end function dcdate_eq_fr

  logical function dcdate_eq_if(i, diff) result(result)
    !
    ! 引数 *diff* の日時差が *i* と等しいかどうかを比較します. *diff*
    ! を秒数に換算した値と *i* とが等しい場合, .true. が返ります.
    !
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    integer, intent(in):: i
  continue
    result = dcdate_eq_rf(real(i), diff)
  end function dcdate_eq_if

  logical function dcdate_eq_rf(r, diff) result(result)
    !
    ! 引数 *diff* の日時差が *r* と等しいかどうかを比較します. *diff*
    ! を秒数に換算した値と *r* とが等しい場合, .true. が返ります.
    !
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    real, intent(in):: r
  continue
    if (real(EvalSec(diff)) == r) then
      result = .true.
    else
      result = .false.
    end if
  end function dcdate_eq_rf

  logical function dcdate_eq_tt(time1, time2) result(result)
    !
    ! 2 つの引数の日時を比較します.
    ! 1 つ目の引数に格納される日時が 2 つ目の引数に格納される日時
    ! と同じ場合, .true. が返ります.
    !
    implicit none
    type(DC_DATETIME), intent(in):: time1, time2
    real(DP):: time1_sec, time2_sec
  continue
    time1_sec = EvalSec(time1) + EvalSec(ZoneToDiff(time1 % zone))
    time2_sec = EvalSec(time2) + EvalSec(ZoneToDiff(time2 % zone))
    if (time1_sec == time2_sec) then
      result = .true.
    else
      result = .false.
    end if
  end function dcdate_eq_tt

  logical function dcdate_gt_ff(diff1, diff2) result(result)
    !
    ! 2 つの引数の日時差を比較します.
    ! 1 つ目の引数に格納される日時差が 2 つ目の引数に格納される日時差
    ! よりも大きい場合, .true. が返ります.
    !
    use dc_date_types, only: CYCLIC_MDAYS
    implicit none
    type(DC_DIFFTIME), intent(in):: diff1, diff2
  continue
    if (EvalSec(diff1) > EvalSec(diff2)) then
      result = .true.
    else
      result = .false.
    end if
  end function dcdate_gt_ff

  logical function dcdate_gt_tt(time1, time2) result(result)
    !
    ! 2 つの引数の日時を比較します.
    ! 1 つ目の引数に格納される日時が 2 つ目の引数に格納される日時
    ! よりも進んでいる場合, .true. が返ります.
    !
    implicit none
    type(DC_DATETIME), intent(in):: time1, time2
    real(DP):: time1_sec, time2_sec
  continue
    time1_sec = EvalSec(time1) + EvalSec(ZoneToDiff(time1 % zone))
    time2_sec = EvalSec(time2) + EvalSec(ZoneToDiff(time2 % zone))
    if (time1_sec > time2_sec) then
      result = .true.
    else
      result = .false.
    end if
  end function dcdate_gt_tt

  logical function dcdate_lt_ff(diff1, diff2) result(result)
    !
    ! 2 つの引数の日時差を比較します.
    ! 2 つ目の引数に格納される日時差が 1 つ目の引数に格納される日時差
    ! よりも大きい場合, .true. が返ります.
    !
    implicit none
    type(DC_DIFFTIME), intent(in):: diff1, diff2
  continue
    result = .not. dcdate_gt_ff(diff1, diff2)
  end function dcdate_lt_ff

  logical function dcdate_lt_tt(time1, time2) result(result)
    !
    ! 2 つの引数の日時を比較します.
    ! 2 つ目の引数に格納される日時が 1 つ目の引数に格納される日時
    ! よりも進んでいる場合, .true. が返ります.
    !
    implicit none
    type(DC_DATETIME), intent(in):: time1, time2
  continue
    result = .not. dcdate_gt_tt(time1, time2)
  end function dcdate_lt_tt

  type(DC_DIFFTIME) function dcdate_mod_ff(diff1, diff2) result(result)
    !
    ! 引数 <b>diff1</b> を <b>diff2</b> で除算した際の余りを返します.
    !
    ! ※ 注意: 月差と日時の混在する除算は近似的結果になるおそれがあります
    !
    use dc_date_types, only: CYCLIC_MDAYS
    implicit none
    type(DC_DIFFTIME), intent(in):: diff1, diff2
    real(DP):: sec1, sec2
  continue
    result % day_seconds = diff1 % day_seconds
    if (diff1 % day == 0 .and. diff2 % day == 0 .and. diff1 % sec == 0.0 .and. diff2 % sec == 0.0) then
      result % mon = mod(diff1 % mon, diff2 % mon)
      result % day = 0
      result % sec = 0.0
    else if (diff1 % sec == 0.0 .and. diff2 % sec == 0.0) then
      result % mon = 0
      result % day = mod((CYCLIC_MDAYS * diff1 % mon + diff1 % day), (CYCLIC_MDAYS * diff2 % mon + diff2 % day))
      result % sec = 0.0
    else
      sec1 = diff1 % day_seconds * (CYCLIC_MDAYS * diff1 % mon + diff1 % day) + diff1 % sec
      sec2 = diff2 % day_seconds * (CYCLIC_MDAYS * diff2 % mon + diff2 % day) + diff2 % sec
      result % sec = mod(sec1, sec2)
      result % day = 0.0
      result % mon = 0.0
      call dcdate_normalize(result % day, result % sec, result % day_seconds)
    endif
  end function dcdate_mod_ff

  type(DC_DIFFTIME) function dcdate_mul_df(factor, diff) result(result)
    !
    ! ※ 注意 : 月差を非整数倍すると近似的結果になるおそれがあります
    use dc_types, only: DP
    use dc_date_types, only: CYCLIC_MDAYS
    implicit none
    real(DP), intent(in):: factor
    type(DC_DIFFTIME), intent(in):: diff
    real(DP):: month, day
  continue
    month = factor * diff % mon
    result % mon = int(month)
    day = factor * diff % day + int(CYCLIC_MDAYS * (month - result % mon))
    result % day = int(day)
    result % sec = factor * diff % sec + (day - result % day) * diff % day_seconds
    result % day_seconds = diff % day_seconds
    call dcdate_normalize(result % day, result % sec, result % day_seconds)
  end function dcdate_mul_df

  type(DC_DIFFTIME) function dcdate_mul_fd(diff, factor) result(result)
    !
    ! ※ 注意 : 月差を非整数倍すると近似的結果になるおそれがあります
    use dc_types, only: DP
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    real(DP), intent(in):: factor
  continue
    result = dcdate_mul_df(factor, diff)
  end function dcdate_mul_fd

  type(DC_DIFFTIME) function dcdate_mul_fi(diff, factor) result(result)
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    integer, intent(in):: factor
  continue
    result = dcdate_mul_if(factor, diff)
  end function dcdate_mul_fi

  type(DC_DIFFTIME) function dcdate_mul_fr(diff, factor) result(result)
    !
    ! ※ 注意 : 月差を非整数倍すると近似的結果になるおそれがあります
    implicit none
    type(DC_DIFFTIME), intent(in):: diff
    real, intent(in):: factor
  continue
    result = dcdate_mul_rf(factor, diff)
  end function dcdate_mul_fr

  type(DC_DIFFTIME) function dcdate_mul_if(factor, diff) result(result)
    !
    ! 日時差 *diff* と *facter* とを乗算した結果を返します.
    !
    implicit none
    integer, intent(in):: factor
    type(DC_DIFFTIME), intent(in):: diff
  continue
    result % mon = factor * diff % mon
    result % day = factor * diff % day
    result % sec = factor * diff % sec
    result % day_seconds = diff % day_seconds
    call dcdate_normalize(result % day, result % sec, result % day_seconds)
  end function dcdate_mul_if

  type(DC_DIFFTIME) function dcdate_mul_rf(factor, diff) result(result)
    !
    ! ※ 注意 : 月差を非整数倍すると近似的結果になるおそれがあります
    use dc_types, only: DP
    implicit none
    real, intent(in):: factor
    type(DC_DIFFTIME), intent(in):: diff
  continue
    result = dcdate_mul_df(real(factor, DP), diff)
  end function dcdate_mul_rf

  type(DC_DIFFTIME) function dcdate_sub_ff(diff1, diff2) result(result)
    implicit none
    type(DC_DIFFTIME), intent(in):: diff1, diff2
  continue
    result % mon = diff1 % mon - diff2 % mon
    result % day = diff1 % day - diff2 % day
    result % sec = diff1 % sec - diff2 % sec
    result % day_seconds = diff1 % day_seconds
    call dcdate_normalize(result % day, result % sec, result % day_seconds)
  end function dcdate_sub_ff

  type(DC_DATETIME) function dcdate_sub_tf(time, diff) result(result)
    !
    ! 2 つの日時 (DC_DATETIME 型) もしくは
    ! 日時差 (DC_DIFFTIME 型)の減算を行います.
    !
    implicit none
    type(DC_DATETIME), intent(in):: time
    type(DC_DIFFTIME), intent(in):: diff
    integer:: time_year, time_mon, time_day, time_caltype
    real(DP):: time_sec
    character(6):: time_zone
  continue
    call Eval(time, year = time_year, mon = time_mon, day = time_day, sec = time_sec, caltype = time_caltype, zone = time_zone)
    call Create(result, year=time_year, mon = time_mon - diff % mon, day = time_day - diff % day, sec = time_sec - diff % sec, caltype = time_caltype, zone = time_zone)
  end function dcdate_sub_tf

  type(DC_DIFFTIME) function dcdate_sub_tt(time1, time2) result(result)
    implicit none
    type(DC_DATETIME), intent(in):: time1, time2
  continue
    result % day = time1 % day - time2 % day
    result % sec = time1 % sec - time2 % sec + EvalSec(ZoneToDiff(time1 % zone) - ZoneToDiff(time2 % zone))
    result % day_seconds = time1 % day_seconds
    call dcdate_normalize(result % day, result % sec, result % day_seconds)
  end function dcdate_sub_tt