When you view the properties of a Microsoft Excel workbook, the Modified date and time may temporarily change even when you make no changes to the file. You may notice this behavior when you open the file in Excel and then click Properties on the File menu, or when you view file details in Microsoft Windows Explorer.
This problem occurs because when you open an Excel workbook, Excel writes the name of the current user to the header of the file. This is necessary so that other users receive the "file in use" notification. The operating system then updates the last modified date.
There is a binary difference between the file contents before you open and close an Excel workbook without saving it and after you open and close an Excel workbook without saving it.
The issue does not apply to the following file formats in Microsoft Office Excel 2007: