This summary is not the rule. The rule text controls.
If you disagree with a judgment and want the court to change or fix it, you have to ask within 30 days after the judgment is entered. This request is called a "motion to alter or amend."
The 30-day deadline is strict. You need to both file the motion with the court and have it served on the other side within that time. If you miss this window, the court will likely refuse to consider your request, and you may lose your chance to fix any errors in the judgment through this type of motion.
This rule exists to keep cases moving forward and give everyone a clear end point. If you think the judge made a mistake, whether about the facts, the law, or new evidence that came up, you need to act quickly and get your motion filed right away rather than waiting.