library(foreign)

time<-read.dta(file="http://kanggc.iptime.org/book/data/chap10-2.dta")

n<-length(time$var1)

s<-sd(time$var1)

t19<-qt(0.025, df=19, lower.tail=F)

average<-mean(time$var1)

LCL<-average-t19*(s/sqrt(n))

UCL<-average+t19*(s/sqrt(n))

LCL;UCL