Passing time zones

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I need some ideas on how I can have the user input the time zone name.
The list has cons cells with the car having a time-zone label, while the
cdr has a name for the TZ Environment Variable.  The user will just select
the time-zone label.

  (interactive
    (list
      (read-string " Clock Name: " "World Clock")

      (let ( (cseq '(("UTC"         . "UTC")
                     ("New York"    . "America/New_York")
                     ("London"      . "Europe/London")
                     ("Tokyo"       . "Asia/Tokyo")
                     ("Sydney"      . "Australia/Sydney")
                     ("Berlin"      . "Europe/Berlin")
                     ("Los Angeles" . "America/Los_Angeles")
                     ("Kolkata"     . "Asia/Kolkata"))) )
        (completing-read " Time Zone: " cseq nil t))

      (let ( (cseq '("red" "green" "blue" "orange" "purple"
                     "cyan" "magenta" "yellow")) )
        (completing-read " Colour: " cseq nil t))))