Are your fields recognised by Tableau as the datetime data type, or are they strings?
If Tableau knows they are datetimes you can use the DATEDIFF() function to find the difference in units of 'day', 'hour', 'minute' or 'second', or you can just subtract the start from the finish and that will give you a floating point number representing the (fractional) number of days between the two (i.e. if the answer is 1.25 that means 1 day and 6 hours).
If your fields are strings you'll need to convert them to datetimes first. You'll need to reformat slightly to get those strings into a format the DATETIME() function recognises (easiest is just to insert a couple of "/" characters.