how to convert start date parameter to microsecond in custom query?

I try to use custom query in Google Looker Studio as the data source. and here is the custom SQL to fetch data from my BigQuery

WITH analytics_data AS (
  SELECT user_pseudo_id, 
         UNIX_MICROS(TIMESTAMP("2023-03-10 00:00:00", "+7:00")) AS start_day,
         3600*1000*1000*24*7 AS one_week_micros
  FROM `project.analytics_297774888.events_*`
  WHERE _table_suffix BETWEEN @DS_START_DATE AND @DS_END_DATE
)

as you can see I enable date range parameters, so I use @DS_START_DATE and @DS_END_DATE . my problem is in this one line

UNIX_MICROS(TIMESTAMP("2023-03-10 00:00:00", "+7:00")) AS start_day

currently it is hardcoded string ("2023-03-10 00:00:00"). I want to replace the date string with @DS_START_DATE. If I change the code to be like one below, I will have error

UNIX_MICROS(TIMESTAMP(@DS_START_DATE, "+7:00")) AS start_day

I am trying to get Unix Microsecond (number of microseconds since 1970-01-01) of date @DS_START_DATE at 00:00:00 in timezone UTC + 7

how to do that?

Since @DS_START_DATE and @DS_END_DATE has a shape of YYYYmmdd, you can parse it using PARSE_TIMESTAMP like below along with timezone.

UNIX_MICROS(PARSE_TIMESTAMP('%Y%m%d', @DS_START_DATE, '+7:00'))

Try this:

 WITH analytics_data AS (
  SELECT user_pseudo_id, 
         UNIX_MICROS(TIMESTAMP(CONCAT(@DS_START_DATE, " 00:00:00"), "+7:00")) AS start_day,
         3600*1000*1000*24*7 AS one_week_micros
  FROM `project.analytics_297774888.events_*`
  WHERE _table_suffix BETWEEN @DS_START_DATE AND @DS_END_DATE
)