[ruby-cvs:78718] d6a2bce64a (master): time.c (find_time_t): fix round-to-zero bug

Yusuke Endoh	2019-12-17 10:31:20 +0900 (Tue, 17 Dec 2019)

  New Revision: d6a2bce64a

  https://github.com/ruby/ruby/commit/d6a2bce64a

  Log:
    time.c (find_time_t): fix round-to-zero bug
    
    `find_time_t` did not work correctly for year older than the Epoch
    because it used C's integer division (which rounds negative to zero).
    
    For example, `TIme.new(1933)` returned a wrong time whose year is 1922
    in Asia/Kuala_Lumpur because there is no 00:00:00 1st Jan. 1933 in the
    time zone.
    
    ```
    $ TZ=Asia/Kuala_Lumpur ruby -e 'p Time.new(1933)'
    1932-12-31 00:00:00 +0700
    ```
    
    This change fixes the issue by using `DIV` macro instead of `/`.
    Now `Time.new(1933)` returns a time in 1933.
    
    ```
    $ TZ=Asia/Kuala_Lumpur ruby -e 'p Time.new(1933)'
    1933-01-01 00:20:00 +0720
    ```
    
    [Bug #16159]

  Modified files:
    time.c