WITH NEXT AS (
  SELECT
        user_id,
        START,
        END,
        COALESCE(DATETIME(LAG(END) OVER (partition BY user_id ORDER BY START), '15 days') < START, 0) AS step
    FROM
        t
),

num AS (
  SELECT
      user_id,
      START,
      END,
      SUM(step) OVER (partition BY user_id ORDER BY START) + 1 AS sub_sequence_num
  FROM
      NEXT
)

SELECT
    user_id,
    START,
    END,
    sub_sequence_num,
    dense_rank() OVER (partition BY user_id, sub_sequence_num ORDER BY START) AS sub_num_in_seq
FROM
    num
ORDER BY
    user_id,
    START