first_time, last_time

first_time(
    cs CounterSummary
) RETURNS TIMESTAMPTZ

last_time(
    cs CounterSummary
) RETURNS TIMESTAMPTZ

WITH t as (
    SELECT
        time_bucket('1 day'::interval, ts) as dt,
        counter_agg(ts, val) AS cs -- get a CounterSummary
    FROM table
    GROUP BY time_bucket('1 day'::interval, ts)
)
SELECT
    dt,
    first_time(cs) -- extract the timestamp of the first point in the CounterSummary
    last_time(cs) -- extract the timestamp of the last point in the CounterSummary
FROM t;