Use a slash or a hyphen to separate the parts of a date in Excel. Excel recognizes information in that form as a date and stores it as a serial number: 8/22/2005 and 22-Aug-2005 are both stored as serial number 38586. Excel will also recognize August 22, 2005 as a date.
Excel can surprise you by changing dates that you've typed in one format into its own default format. For example, if you typed August 22, 2005, Excel might change the format to 22-Aug-05 or to another default format. Regardless of the format, the date's serial number is 38586. Unless, that is, you use a format that Excel doesn't recognize as a date.
For example, if you typed August 22 2005 or 8,22,05, Excel would store that information as plain text, not as 38586.
To see some of the other ways you can use slashes or hyphens to format dates, you can open the Format Cells dialog box shown in the picture.