For better result I would like to suggest you to do the followings,
Format both the Cells for DD/MM/YY HH:MM:SS.
Format the Formula Cell as HH:MM:SS.
Suppose in Cell A2 Start Date & Time is 20/11/2017 23:05 and in B2 End Date & Time is 21/11/2017 01:35.
Write the Formula in D2 ,
The answer I got is 01:34:59.
NB: To make the combination with your suggest Time, I've added recent dates also because, in case of more than 1 day's difference Excel will not able to calculate the real time difference.
My Formula calculates accurately rather than other.
Like if you use Formula = INT((B2-A2)*24) the answer will 2, [when the Cell Format is General & if you covert it into HH:MM you get 00:00]
Hope this help you, I've posted the solution after is been tested by me.