|What you are seeing are "date serial numbers".|
This site will explain how Excel stores internally, which will help explain what you are seeing - but not how to fix it. That comes later.
Excel often gets locked into certain formats when dates are involved, usually as a result of how the dates ended up in the spreadsheet. Most of the time the problem is caused when the dates are downloaded or copied from a website.
Unfortunately, there isn't a single solution to fixing these date problems since we can't tell what Excel is "confused" about internally.
One thing you could try is this:
Let's say your 5 digit numbers start in A1.
In a blank column enter this formula and drag it down:
This should display a date in your default date format.
Then do a Copy...PasteSpecial...Values on that column to eliminate the formulas and leave the dates.
Copy these dates over your column with the serial numbers and then delete the temporary column.
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