The class you're looking for in POI is DataFormatter
When Excel writes the file, some cells are stored as literal Strings, while others are stored as numbers (including dates). For the latter, a floating point value representing the cell is stored in the file, so when you ask POI for the value of the cell that's what it actually has.
Sometimes though, especially when doing Text Extraction (but not always), you want to make the cell value look like it does in Excel. It isn't always possible to get that exactly in a String (non full space padding for example), but the DataFormatter class will get you close.
Also note that in Excel, dates are stored as floating point numbers since ~1900, which is why you're seeing a number not a date. Use DataFormatter (or similar) to have it rendered as a date
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