The student who thinks they can prep for the Excel 2019 MOS exam in a day is probably right, and that should worry them more than it reassures them. Twenty to thirty hours of self-learning is not nothing, but it is also not the kind of depth that typically survives a certification test. The linked Microsoft page lays out a clear set of objectives, and while charts and formulas look straightforward on a list, the exam is not asking you to recall what a VLOOKUP does. It is asking you to perform it under pressure, with no one watching, and with the clock running. That is a different skill entirely.
What the student is really underestimating is the gap between familiarity and fluency. You can watch a few videos on pivot tables and feel like you have seen the material. Then the test gives you a messy dataset and asks you to clean it, group it, and summarize it in a way that matches a specific output. If you have only touched these features a handful of times, you will hesitate. Hesitation is what kills test scores. The exam is not designed to trip you up with trick questions. It is designed to measure whether you can work efficiently, not just correctly. A day of review might get you through the basics, but it will not build the muscle memory that separates a passing score from a near miss.
That said, the student is not wrong to feel optimistic. The exam is free through their school, which removes the biggest barrier to entry. The cost of failure is low, so there is a real argument for just trying it. But the smarter play is to treat this as a diagnostic, not a finish line. Take a practice assessment, see where you actually stumble, and spend your day targeting those weak spots. If you already know your way around formulas and formatting, charts might indeed be a quick review. But if you have never built a table or used conditional formatting, no amount of last-minute cramming will save you.
The honest answer is that a day might be enough, but only if you already know more than you think you do. If your 20 to 30 hours were spent actually building spreadsheets, not just clicking through menus, you are closer than you realize. If those hours were scattered and unstructured, you are likely overconfident. The exam is not impossibly hard, but it is also not a formality. Use the free practice tool, time yourself, and be brutally honest about your results. Then decide if a day is truly enough or if you need to invest a few more focused sessions. The test will still be there. Your time is the only thing you cannot get back.