Excel is not the only thing that opens one, and for reading a sheet it is rarely the quickest.
An `.xlsx` is a zip archive of XML, the same construction as a `.docx`: sheets, styles and shared strings as separate files inside one compressed folder. It holds types, formulas, formatting and several sheets at once, which is everything a CSV cannot.
Google Sheets imports one through File, Import. LibreOffice Calc opens it on any desktop system and is free. Apple Numbers opens one on a Mac, and Microsoft’s own web version of Excel reads one without a paid licence.
A cell showing `####` is a column too narrow to display the value, not a broken file. A date that reads as a five-digit number is the underlying serial value with the formatting lost.
Formulas are stored alongside their last computed result. A file opened in something that does not evaluate them shows the results, which are correct as of whenever the file was last saved and not necessarily now.
The usual route is exporting each sheet to CSV and converting that, which loses everything but the active sheet. Converting the workbook directly gives you a Markdown table per sheet, with a table of contents when there is more than one.