You could enter this formula in E3:

=IF(B3D2,B3-D2,0)

Format the cell as time and then copy down column E for as many data
items as you have.

You can then sum column E in another cell and format that cell using a
custom format of [h]:mm so that it displays hours above 24.

I have assumed that the data always occurs for the same date - post
back if this is not the case and explain if 3/1 represents 3rd January
or 1st March. Is this text, or a custom date format?

Hope this helps.

Pete