Hi Daniel1973,
According to your description, what your purpose is to convert the text string to a date before insert into Excel.
I have made a flow for your scenario, please check it for a reference.
As you say the options are like: "01.01.2019" "01.02.2019"
First, I use the Compose action to split the String, the expression is : split(body('Get_response_details')?['Month'],'.')
Second, I use the Compose action to convert the string to Date format., the expression is : concat(outputs('Compose')[2],'/',outputs('Compose')[1],'/',outputs('Compose')[0])


Please feel free post back if you need more help.
Best regards,
Zhonys