A few things that need correction there.if earth is slowing down then our day should be not 24 hours . some second should be decreased every year. but we know that our days are same.
Your day is Not 24 hours. Its 23 hours 56 minutes and 4 seconds. Earth's rotation has been slowing down for millions of years. Atomic clocks show that today the day is longer by 1.7 milliseconds than what it was 100 years ago.
Also if the Earth rotation is slowing down your day will NOT be shorter, but Longer.