あるべき答えがなにかは知らんが、手元でやってみた。
$ jruby -v
jruby 1.7.18 (1.9.3p551) 2014-12-22 625381c on Java HotSpot(TM) 64-Bit Server VM 1.8.0_25-b17 +jit [linux-amd64]

ENV['TZ'] = 'US/Pacific'
p Time.local(1975, 1, 1).isdst # => true
p Time.local(1975, 2, 1).isdst # => true
p Time.local(1975, 3, 1).isdst # => true
p Time.local(1975, 4, 1).isdst # => true
p Time.local(1975, 5, 1).isdst # => true
p Time.local(1975, 6, 1).isdst # => true
p Time.local(1975, 7, 1).isdst # => true
p Time.local(1975, 8, 1).isdst # => true
p Time.local(1975, 9, 1).isdst # => true
p Time.local(1975, 10, 1).isdst # => true
p Time.local(1975, 11, 1).isdst # => true
p Time.local(1975, 12, 1).isdst # => true
ENV[' TZ '] = nil

# 実行結果
false
false
true
true
true
true
true
true
true
true
false
false