Note that $\Pr(X\le31) = \Pr(X<32).$ So do you use $\dfrac{31-30}{\sqrt{30}}$ or $\dfrac{32-30}{\sqrt{30}}\text{?}$
They are both approximations. In this situation just using the number halfway between those -- that is called a "continuity correction":
$$
\Phi\left( \frac{ 31.5 - 30}{\sqrt{30}} \right) \approx 0.6079.
$$
Note that $\Pr(X\le31) = \Pr(X<32).$ So do you use $\dfrac{31-30}{\sqrt{30}}$ or $\dfrac{32-30}{\sqrt{30}}\text{?}$
They are both approximations. In this situation just using the number halfway between those -- that is called a "continuity correction": $$ \Phi\left( \frac{ 31.5 - 30}{\sqrt{30}} \right) \approx 0.6079. $$