But by adding another year on, the internship looked a year longer than it actually was. That is a significant error.
It is up to you. However, I would tell them at the earliest opportunity given the nature of the error just in case you are offered. If you wait until post offer it could look like you have intentionally lied/held this information back. If you were at an earlier stage of the process, I would wait to hear back if you were successful in getting to the next stage, but this sounds like the next stage is an offer, and so it’s best to tell them sooner rather than later.