Because dates are stored as numbers in Excel
In the example below 43190 in cell A3 is 3/31/18 formatted as a number.
From Microsoft Office Support:
https://support.office.com/en-us/article/date-function-e36c0c8c-4104-49da-ab83-82328b832349
Note: Excel stores dates as sequential serial numbers so that they can be used in calculations. January 1, 1900 is serial number 1, and January 1, 2008 is serial number 39448 because it is 39,447 days after January 1, 1900. You will need to change the number format (Format Cells) in order to display a proper date.
<tbody>
[TD="align: center"]1[/TD]
[TD="align: right"]3/31/2018[/TD]
[TD="align: right"]14[/TD]
[TD="align: center"]2[/TD]
[TD="align: center"]3[/TD]
[TD="align: right"]43190[/TD]
</tbody>
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