Here's the controlling issue for adding timezone support to Drupal core. As of now, August 1st, 2017, it doesn't appear to be ready.

Instead, I just added a timezone field to the node using the Timezone module, then appended it to the date range on field preprocess.

This won't work for you if you have multiple dates on an entity with different timezones (you might want to try the patch linked above), but it will fit most use cases.