Consider the problem,

In the case of mineral, coverage area (r) increases

Therefore, Inertia \(I = Mr ^2\)

If \(r\) increases, then \(I\) increases

and, also \(L = I \omega\)

\(L\) is conserved, then if \(I\) increases, \(\omega\) decreases.

And also, \(\omega=\frac{2 \pi}{T}\)

If \(\omega\) decreases, then Time period ( \(T\) ) increases.

Therefore, length of the day increases.