Hi @Anonymous
Thank you for posting.
Excel stores internally the dates in a number format, so to use them in flow need to convert that integer to a date.
I created a simple flow and can create event that End time is 14 days after Due Date.
You can refer to the screenshots as below:

Use the following formula to get the correct date from excel.
Expression: formatDateTime(addDays('1899-12-30',int(items('Apply_to_each')?['Due Date'])),'yyyy-MM-dd')

Use the following formula to fill in the date that is 14 days after Due Date in the End time
Expression: addDays(outputs('Compose'),14,'yyyy-MM-dd')

Then save Flow and can test it.
This is my Excel

After Flow runs, two event will be created as below:

Hope the content above may help you.
Best Regards
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