Hi @LZA
Thank you for your responding.
If the Date in the automated mail is always in format dd/MM/yyyy and you can't change the format. You can split the date from email and than regroup it.
You could refer to screenshot below to modify the flow.
Insert Compose named Day under Start , the expression in the Day as below:
first(split(outputs('Start'),'.'))

The expression in the Month as below:
first(skip(split(outputs('Start'),'.'),1))

The expression in the Year as below:
last(split(outputs('Start'),'.'))

The expression in the Start date field as below:
concat(outputs('month'),'/',outputs('day'),'/',outputs('year'))

Use the same method to get the End date

Then save Flow and can test it. When receive the email as below:

The calendar can be created as below:

You can refer to this document to see how these functions are used.
https://docs.microsoft.com/en-us/azure/logic-apps/workflow-definition-language-functions-reference
Hope the content above may help you.
Best Regards
Anna