Hi Charlotte Mueller,
First thing is you will have to add a new Attribute for to show in your Desired format. While calling the Dataview Page, perform your Conversion in microflow and then set the Value in the Attribute you have created and show the page.
For Conversions you can make use of String functions as Roy Mentioned.
it seems like, you want to convert string into date time. for this you can use parsedatetime() to convert string into date in microflow.
To display or select date and time in dataview, please refer below link and download react date time picker widget and use it in dataview-
Or use a substring to find the correct values (according if the source string (PT08H00M00S) remains the same)
Determine the position of the first number, take 2 symbols, store it in a variable and keep doing the same until you have all the values you need and combine those to a new string/attribute
edit: I'm not sure if parsing will work, since there are also letters involved in this case