To recompute the 95% CL Mean from the example:
17.37585 ; 23.50713

/* to find the tCrit here -> 1.984217*/
proc iml;
alpha = 0.05;
n=100;
tCrit = quantile("T", 1 - alpha/2, n-1);
print(tCrit);
quit;

geomean / exp(tCrit * SEM)
geomean * exp(tCrit * SEM)

95% CL Mean from the example:
17.37585 =
20.2103 / exp( 1.984217 * log(2.1416283)/10 )

23.50713 =
20.2103 * exp( 1.984217 * log(2.1416283)/10 )