I need to find out if there is a way to know how when a specific entry was made in an excel sheet. A log that states date and time.

Previous entries, not unless you've turned on the "track changes" feature. From the Excel 2003 help file:

1. On the Tools menu, click Share Workbook, and then click the Editing tab.
2. Select the Allow changes by more than one user at the same time check box.
3. Click the Advanced tab.
4. Under Track changes, click Keep change history for, and in the Days box, type the number of days of change history

(change history: In a shared workbook, information that is maintained about changes made in past editing sessions. The information includes the name of the person who made each change, when the change was made, and what data was changed.) that you want to keep.

Be sure to enter a large-enough number of days because Microsoft Excel permanently erases any change history older than this number of days.

5. Click OK, and if prompted to save the file, click OK.