DateTime.pmが１秒ずれる

add_duration()を高速化するべく、_handle_offset_modifier()をCで書き直してるのだが１秒ずれる。

t/19leap_second.........NOK 121                                              

#   Failed test 'UTC time end of leap second day'

#   in t/19leap_second.t at line 750.

#          got: '1997-07-01T22:59:58'

#     expected: '1997-07-01T22:59:59'

t/19leap_second.........NOK 144                                              

#   Failed test 'dt is 2006-01-01T00:00:00'

#   in t/19leap_second.t at line 893.

#          got: '2005-12-31T23:59:60'

#     expected: '2006-01-01T00:00:00'

t/19leap_second.........NOK 146                                              

#   Failed test 'dt is 2006-01-01T00:00:00'

#   in t/19leap_second.t at line 908.

#          got: '2005-12-31T23:59:60'

#     expected: '2006-01-01T00:00:00'

# Looks like you failed 3 tests of 148.

t/19leap_second.........dubious                                              

        Test returned status 3 (wstat 768, 0x300)

DIED. FAILED tests 121, 144, 146

        Failed 3/148 tests, 97.97% okay

Leap second のハンドリングが変なんだろうけど、どこだ一体・・・