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** ,

=TIME(HOUR(B2),MINUTE(B2),SECOND(B2)-TIME(HOUR(A2),MINUTE(A2),SECOND(A2)))

**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.**

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