-- Before UNIX epoch
WITH
    toDateTime64('1959-09-16 19:20:12.999999998', 9, 'UTC') AS dt1,
    toDateTime64('1959-09-16 19:20:12.999999999', 9, 'UTC') AS dt2
SELECT
        dt1 < dt2,
        (dt1 + INTERVAL 1 NANOSECOND) = dt2,
        (dt1 + INTERVAL 2 NANOSECOND) > dt2,
        (dt1 + INTERVAL 3 NANOSECOND) > dt2;

-- At UNIX epoch border
WITH
    toDateTime64('1969-12-31 23:59:59.999999998', 9, 'UTC') AS dt1,
    toDateTime64('1969-12-31 23:59:59.999999999', 9, 'UTC') AS dt2
SELECT
        dt1 < dt2,
        (dt1 + INTERVAL 1 NANOSECOND) = dt2,
        (dt1 + INTERVAL 2 NANOSECOND) > dt2,
        (dt1 + INTERVAL 3 NANOSECOND) > dt2;

-- After UNIX epoch
WITH
    toDateTime64('2001-12-31 23:59:59.999999998', 9, 'UTC') AS dt1,
    toDateTime64('2001-12-31 23:59:59.999999999', 9, 'UTC') AS dt2
SELECT
        dt1 < dt2,
        (dt1 + INTERVAL 1 NANOSECOND) = dt2,
        (dt1 + INTERVAL 2 NANOSECOND) > dt2,
        (dt1 + INTERVAL 3 NANOSECOND) > dt2;

-- At upper DT64 bound (DT64 precision is lower here by design)
WITH
    toDateTime64('2282-12-31 23:59:59.999998', 6, 'UTC') AS dt1,
    toDateTime64('2282-12-31 23:59:59.999999', 6, 'UTC') AS dt2
SELECT
        dt1 < dt2,
        (dt1 + INTERVAL 1 MICROSECOND) = dt2,
        (dt1 + INTERVAL 2 MICROSECOND) > dt2,
        (dt1 + INTERVAL 3 MICROSECOND) > dt2;