Question: how to calculate time difference in excel if more than 24 hours?
Answer: In cell B2 type 12:45, and in cell B3 type 15:30.
Type =B2+B3 in cell B4, and then press Enter. ... 
To display the time as more than 24 hours, select cell B4.
On the Home tab, in the Cells group, choose Format, and then choose Format Cells.