The Earth slowing down will cause the days to get longer, not shorter. The reason why this is happening is because of the tides transferring the Earth's rotational momentum to the moon as it orbits the Earth, causing the moon to orbit farther out. When the length of the day on the Earth is the same as the period of the orbit of the moon, then the Earth will stop slowing down.

Interesting question! The reason the Earth’s orbit is slowing down is due to the Moon. Through a phenomena called tidal acceleration, momentum and energy are transferred between the Earth and Moon through tidal friction that slows down the Earth’s orbit speed. This results in an increase in the (solar) day and also increases the (lunar) month. However, eventually there will be a syncing of tidal forces which ultimately means the Earth and Moon rotate always facing the same spot. At this point, billions of years in the future, the solar day will equal the lunar month, approximately 47 current days. A future observer would see a 47 day-long “day”, while the moon remains permanently fixed in the sky.