Here's how to perform data validation for dates and times in Excel:

1. Select the Cells:

- Choose the cell(s) where you want to restrict data entry to dates or times.

2. Access Data Validation:

- Navigate to the "Data" tab on the Excel ribbon.
- In the "Data Tools" group, click "Data Validation."

3. Set Allow Option:

- In the "Settings" tab of the Data Validation window, under "Allow," choose the appropriate option:
- For Dates: Select "Date" from the dropdown menu.
- For Times: Select "Time" from the dropdown menu.

4. Define Criteria (Optional):

- With "Date" or "Time" selected, you can further define data entry restrictions using the "Criteria" section:
- Between: Specify a start and end date/time range for valid entries.
- Equal to: Allow only entries that match a specific date/time.
- Greater than/Less than: Restrict entries to be after or before a particular date/time.
- Leave blank: No additional criteria, only allows dates/times in general.

5. Input Message (Optional):

- In the "Input Message" tab, you can create a custom message that appears when users select the cell. This message can explain the expected data format (e.g., "Enter date in YYYY-MM-DD format").

6. Error Alert (Optional):

- In the "Error Alert" tab, you can choose to display a custom error message or use the default Excel error message when invalid data is entered.

7. Click OK:

- Once you've set your desired options, click "OK" to apply the data validation rules to the selected cells.

Using this process, you can ensure that users only enter valid dates or times in your specified cells, improving data accuracy and consistency in your Excel worksheets.