Here's how to restrict data entry to specific times using data validation in Excel:

Method 1: Using a List of Allowed Times

1. Create your list: In a separate area of your worksheet, type a list of all the allowed times you want users to be able to enter (e.g., "8:00 AM", "9:00 AM", "10:00 AM", etc.).
2. Select the cell(s): Choose the cell(s) where you want to restrict data entry.
3. Data Validation: Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.
4. Set Allow to List: In the "Settings" tab, under "Allow," choose "List" from the dropdown menu.
5. Define the Source: Click the small dropdown arrow next to the "Source" box and select the cell range containing your list of allowed times.
6. In-cell dropdown (Optional): Check this box if you want a dropdown list to appear within the cell for easy selection.
7. Click OK: Click "OK" to activate data validation.

Method 2: Using a Custom Formula

1. Select the cell(s): Choose the cell(s) where you want to restrict data entry.
2. Data Validation: Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.
3. Set Allow to Custom: In the "Settings" tab, under "Allow," choose "Custom" from the dropdown menu.
4. Formula: In the "Formula" box, enter the following formula:

```excel
=TIME(A1:A1)>=TIME("08:00:00") AND TIME(A1:A1)<=TIME("17:00:00")
```

Explanation of the Formula:

- `TIME(A1:A1)`: This part extracts the time value from the cell you selected (replace A1 with your actual cell reference if different).
- `>=TIME("08:00:00")`: This checks if the extracted time is greater than or equal to 8:00 AM (modify "08:00:00" to your desired start time).
- `AND`: This combines the two conditions.
- `<=TIME("17:00:00")`: This checks if the extracted time is less than or equal to 5:00 PM (modify "17:00:00" to your desired end time).

Note: You can adjust the times in the formula to fit your specific allowed time range.

5. Error Message (Optional): You can set an informative error message to display if someone enters an invalid time. Click on the "Input Message" tab and type your message (e.g., "Please enter a time between 8:00 AM and 5:00 PM").
6. Click OK: Click "OK" to activate data validation.

With either method, users will now be limited to entering times from your allowed list or those that fall within the specified time range in your formula.