Final answer:
When "October 1, 2021" is entered into Excel, it defaults to a date format. Excel identifies the pattern as a date and converts it to a numeric value that represents the date internally while displaying it in a familiar date format to the user.
Step-by-step explanation:
When you enter the date "October 1, 2021" into a cell in Excel, the default format is c. Date. Excel automatically recognizes this as a date because it is a series of numbers separated by slashes, dashes, or, in this case, space and a comma, which conforms to a recognizable date pattern. Excel then formats it internally as a numeric value that represents the date, but displays it to the user in a date format.
For instance, if you input "10/01/2021" into a cell, Excel will interpret this as the 1st of October, 2021, and depending on your regional settings, it may display it as "10/01/2021", "01/10/2021", or in another date format. However, behind the scenes, Excel stores this date as the numeric value '44493', which represents the number of days since January 0, 1900, the starting date from which Excel's date system begins counting.