Here's how to use data validation to restrict data entry to a specific time format (hh:mm) in Excel:

1. Select the cells: Choose the cell(s) where you want to enforce the time format validation.

2. Go to Data Validation: Navigate to the "Data" tab on the ribbon. In the "Data Tools" group, click "Data Validation."

3. Set Allow to Time: In the "Settings" tab of the Data Validation window, under "Allow," choose "Time" from the dropdown menu.

4. Optional: Set Criteria (Between):
- While "Time" as the Allow option generally restricts entries to a valid time format, you can add an extra layer of control by setting a specific time range using "Between."
- Click on the dropdown menu under "Criteria" and choose "Between."
- In the "Start time" box, enter the earliest valid time you want to allow (e.g., "08:00").
- In the "End time" box, enter the latest valid time (e.g., "17:00").

5. Ignore blank (Optional):
- Check this box if you want to allow users to leave the cell empty.

6. Input Message (Optional):
- You can enter a custom message that appears when users select the cell. This message can explain the expected time format (e.g., "Enter time in hh:mm format").

7. Error Alert:
- Choose the type of error alert you want to display if an invalid entry is attempted.
- "Stop" prevents further data entry until a valid time is entered.
- "Warning" allows users to continue with an invalid entry but displays an alert message.
- In the "Error message" box, you can enter a custom message explaining the error (e.g., "Invalid time format. Please use hh:mm").

8. Click OK: Once you've set your desired options, click "OK" to activate the data validation rule.

Now, when users try to enter data in the selected cell(s), Excel will only accept values in the hh:mm format (or within the specified range if you used "Between"). Invalid entries will trigger the chosen error alert.