Shouldn't each  day be one second longer every 1.5 years, if the Earth is rotating slower? Assuming your info is accurate, (1 sec)/(1.5 years) * (4.5 billion years) = The Number of Seconds Shorter The Day Was 4.5 billion years ago. Subtract that from the number of seconds in a day now. I would convert the final answer to minutes.

But your numbers are wrong and lead to an absurd result. "The average day has grown longer by between 15 millionths and 25 millionths of a second every year" http://www.popsci.com/jessica-cheng/article/2008-09/ive-heard-earths-rotation-slowing-how-long-until-days-last-25-hours 

So, do the same calculation with these numbers instead.