If you work with Excel with any regularity at all, it's immediately 100% clear that this is a Date issue. I can't emphasize enough how obvious and elementary this "mystery" is.
MS Excel internally represents dates as a floating point number from epoch of Jan 1 1900
So "40335" or "40335.0" is +40,335 days from that epoch -- which is June 10 2006.
An experienced Excel user often runs into "mysterious 5 digit numbers" from 3xxxx to 4xxxx and will instinctively think it's probably a date that's inadvertently formatted/interpreted as a number. By switching the formatting of that cell to "Date" (Excel right-click "Format cells..." choose Category "Date"), the mystery number becomes a readable mm/dd/yyyy.
Same idea as experienced programmers coming across a 10-digit number like "1602930678" in a JSON or XML data file and instinctively assume it's probably a UNIX timestamp rather than a phone number with area code (160).
These consultants exist to serve that niche.
Good thing Excel doesn't use Unix time... payroll would likely have just written the guy a $1.6 billion check. Look at that, Excel just reduced the company's expenses by 99.997%!