When do I use a z-score vs a t-score for confidence intervals? I have a set of 1000 data points.  I would like to estimate their mean using a confidence interval.  I read somewhere that if the sample size, $n$, is bigger than 30 you should use a t-score, and else use a z-score.
Is that true?

If you don't know the variance of the population, then you should formally always use the $t$-distribution. If you do know the population variance, you can use the standard normal distribution.
However, as $n \to \infty$, the $t$-distribution becomes the same as the standard normal distribution. Even for relatively small samples, the distributions are virtually the same. Therefore, it is common to approximate the $t$-distribution using the normal distribution for sufficiently large samples (e.g. $n>30$ as you indicate).
