With an optional non-interactive parameter:
dpkg-reconfigure -f noninteractive tzdata
To see what is set for the hardware clock:
# cat /etc/adjtime 0.000000 1509214398 0.000000 1509214398 UTC
As a note, the operating system seems to give a container the time in localtime rather than UTC, so the time zone in the container will need to match that of the host. For more info, see adjust timezone to the host at creation time #47, with even more detailed info at Wrong 'ps' process start times shown inside containers #3517.
Current time zone:
# cat /etc/timezone Atlantic/Bermuda