Google BigQuery: How to convert a Date / Timestamp to a different timezone

Use the cast function

TIMESTAMP(datetime_expression, timezone)
DATE(timestamp_expression,timezone)

Examples:

SELECT
  CAST(TIMESTAMP("2008-12-25 15:30:00", "America/Los_Angeles") AS STRING) AS timestamp_str,
  CAST(TIMESTAMP(DATE "2008-12-25", "America/Los_Angeles") AS STRING) AS timestamp_date,
  CAST(TIMESTAMP(DATETIME "2008-12-25 15:30:00", "America/Los_Angeles") AS STRING) AS timestamp_datetime;
+------------------------+------------------------+------------------------+
| timestamp_str          | timestamp_date         | timestamp_datetime     |
+------------------------+------------------------+------------------------+
| 2008-12-25 23:30:00+00 | 2008-12-25 08:00:00+00 | 2008-12-25 23:30:00+00 |
+------------------------+------------------------+------------------------+

SELECT
  DATE(2016, 12, 25) as date_ymd,
  DATE(TIMESTAMP "2016-12-25 05:30:00+07", "America/Los_Angeles") as date_tstz;
+------------+------------+
| date_ymd   | date_tstz  |
+------------+------------+
| 2016-12-25 | 2016-12-24 |
+------------+------------+