Include the date/time value for Last Modified Date or Created Date in a report

The workaround is to create a formula field of type "Date/Time" on the field that is only displaying the date. Perform the following steps to create a new formula field in Lightning:
 

1. Navigate to Setup | Object Manager.
2. Select the desired object.  (In the screenshot below, the Opportunity object was selected)
3. Select Fields & Relationships and click New to create the new field.
4. Select the Data Type Formula and click Next.
5. Name your new field, select the Formula Return Type as Date/Time, and click Next.
6. In the formula editor, click Insert Field and locate the desired field such as Created Date or Last Modified Date and click Insert.
    
   
7. Finish creating the field to add a Description and Help Text, then click Next.
8. Assign the field-level security desired and click Next.
9. Click Save.
10. Create a new report or edit your report to add this new field.
11. If necessary, add the new field to the layouts of custom report types.