@lchap although it's still not just a question of selecting it from the dynamic content box, you can get to the Start time and Completion time using a couple of Send an HTTP request to SharePoint actions and use them in subsequent actions.
You will need your tenant ID. I always get this from Power Apps (select gear, session details then copy the tenant ID). You will also need the ID for the form.
In this example we are going to get all the responses from a form where the user selects a Caribbean country, create an Excel file in SharePoint and from that file select just those responses where Aruba was selected and add those responses into a SharePoint list including the Start time and Completion time.
The flow is triggered manually. Next add a Get my profile (v2) because we'll need my ID. Next add a Send an HTTP request to SharePoint which I've renamed to FormInfo:
- For the site address use a custom value of https://forms.office.com/
- Method: GET
- Uri: formapi/api/{Tenant ID}/users/{Your ID]}/light/forms('Form ID')

Next, add another Send an HTTP request to SharePoint which I've renamed to FormResponses:
- For the site address use a custom value of https://forms.office.com/
- Method: GET
- Uri: formapi/DownloadExcelFile.ashx?formid={Form ID}&minResponseId=1&maxResponseId=
and then add the expression body('FormInfo')?['rowCount']
Next, add a create file action and select the SharePoint site and the folder path. We want the filename to be unique so for the File Name field enter the expression utcNow() in the dynamic content box and select OK then append .xlsx
For the File Content field select Body from the FormResponses section of the dynamic content box.

Next, add a list rows present in a table action. For the ID field select Id from the Create file section of the dynamic content box. Type in Table1 as a custom value in the Table field. Show the advanced options and in the Date/time format select ISO8601.

Next, to just get the items where the country is Aruba, add a Filter array action, select value from the List rows... section of the dynamic content box. Select the column from the spreadsheet containing the country names - is equal to - Aruba

Finally, add an Apply to each and select Body from the Filter array section of the dynamic content box.
Inside the apply to each add a create item action, select the site and list and for each of the columns in the list add an expression of, for example, item()?['Start time']
The completion time would be in the format: item()?[Completion time']

When the flow runs the file is created as shown below:

The 3 rows where Aruba has been selected have been added to the SharePoint list including the Start time and Completion time.

Rob
Los Gallardos
Principal Consultant, SharePoint, Forms and Power Platform, WSP Global (and classic 1967 Morris Traveller driver)