string->date doesn't consider timezone properly

[Takashi Kato]

This should return timezone offset 3600 but 7200 in Amsterdam

(string->date "2016/3/26 15:00:00" "~Y/~m/~d ~H:~M:~S")
;; -> #<date 2016/03/26 15:00:00.000000000 (7200)>

[Takashi Kato]

Considering when on string->date (Fixing #156)

→ <<cset 158ac6bb5879>>