Calculate confidence interval in R - Normal distribution

# CI = xbar +/- t0.975*SE

xbar <- 12 # Assume mean of 12
stddev <- 3 # Standard deviation of 3
n <- 30 # 95 percent confidence interval so right tail at 0.975

error <- qnorm(0.975)*stddev/sqrt(n)
lower_limit <- xbar - error
lower_limit
## [1] 10.92648
upper_limit <- xbar + error
upper_limit
## [1] 13.07352