The stroboscope and air siren were invented only in the 1830s, though both would have been technically feasible since antiquity. The seconds pendulum was invented by Galileo in the late 16th/early 17th century, and was used by Mersenne to achieve the first determination of the actual frequency of an organ pipe (84Hz IIRC) using a very long monochord tuned to the pipe, then doubling its length until he could count the cycles per second against a pendulum.
But the actual value of the second was subject to many errors- it's simple enough to say there are 86400 seconds in a day, but measures of the day (sunrise to sunrise, positions of stars etc.) vary with season, as well as vagaries of the Earth's rotation. That was why it was It would be correct, though, to say that the second has been measured with a resolution of a part in a million since Harrison's time.