If all of Earth's ice melts and flows into the ocean, what would happen to the planet's rotation?

Melting land ice, like mountain glaciers and the Greenland and Antarctic ice sheets, will change the Earth’s rotation only if the meltwater flows into the oceans. For example, if the Greenland ice sheet were to completely melt and the meltwater were to completely flow into the oceans, then global sea level would rise by about seven meters (23 feet) and the Earth would rotate more slowly, with the length of the day becoming longer than it is today, by about two milliseconds.

Melting sea ice, such as the Arctic ice cap, does not change sea level because the ice displaces its volume and, hence, does not change the Earth’s rotation.