In fact the amount they're slowing down by is so unnoticeable that we don't need to worry about it on a year to year or even century to century basis. But we can tell it's slowing over millions of years. Since then the spin has slowed so that the day is 24 and a bit hours long. We know the day was shorter in Jurassic times because coral keeps a record of how much it grows. Just like trees have rings, coral has lines showing when it grew and by how much.
Coral grows during the day when it is sunny.
If the days are longer, there will be more hours of sunlight and the coral will grow more. Coral also lasts a very long time so we can look at coral growing now that was also around 65 million years ago and see how the length of day has changed.