The formulas use the biased (\(1/n\)) sample moments, without small-sample correction, as in package 'moments' (other software may use bias-corrected versions):
Skewness:
$$\frac{\frac{1}{n} \sum_{i=1}^n (x_i-\overline{x})^3}{\left(\frac{1}{n} \sum_{i=1}^n (x_i-\overline{x})^2\right)^{3/2}}$$
(excess) Kurtosis:
$$\frac{\frac{1}{n} \sum_{i=1}^n (x_i - \overline{x})^4}{\left(\frac{1}{n} \sum_{i=1}^n (x_i - \overline{x})^2\right)^2} - 3$$