@sil best guess: same as originally happened. Possible alternative: powerful long wave transmissions from shore(s) counting pips on each GMT hour, radio receivers on ships.
@alex huh, longwave radio. Can you do that with 1700s tech, even if you know how? I know there were crystal spark radios that kids could make in the 1930s out of stuff they had, so it feels possible, but I don't know what you need to have decent broadcast power and whether you can do that with batteries you make out of clay jars and lemon juice