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).