SELECT
  dt as date_A,
  -- find the previous date_B // LAG
  MAX(CASE WHEN x = 2 THEN dt end) OVER (ORDER  BY dt, x ROWS UNBOUNDED PRECEDING ) AS prev_date,
  -- find the next date_B // LEAD
  MIN(CASE WHEN x = 2 THEN dt end) OVER (ORDER  BY dt, x ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING ) AS next_date,
  -- now find the nearest date
  CASE WHEN prev_date IS NULL THEN next_date
       WHEN next_date IS NULL THEN prev_date
       WHEN dt - prev_date < next_date - dt THEN prev_date
       ELSE next_date
  END AS date_B
FROM
 ( -- get all dates, maybe UNION INSTEAD OF UNION ALL
   SELECT date_A as dt, 1 AS x
   FROM table_A
   UNION ALL
   SELECT date_B, 2
   FROM table_B
 ) AS dt
QUALIFY x = 1 -- only rows from table_A