How to calculate no. of hours based on a set of data?

Hi all,

Hope all are well.

In attached file, I am trying to calculate no. of hours in column AE where start and end columns are D and O respectively, however, i don't get correct results. For. Ex row 49.

My end requirement is in Sheet 1 where i want to know total no. of hours against values in column F per month.

Can someone please help? Thanks in advance.

Note: If no value in column O, then column AE should also be blank.