Dragging a formula problem (Needs skip a certain column)

Hello All,

I'm wondering if there's an easier way to drag a formula where there is an alternating consistent column that needs to be skipped? I've attached an example file but here is the general idea that I was hoping to achieve.

I need to calculate the time difference between D4 against B4 which is (D4-B4) and this is shown in M4 and need to have it repeated on F4 against D4 (F4-D4) shown in N4.

How do I drag it easily across the next fields.

Please see the attached file.

Thank you so much!

One way.
Using INDEX formula and manipulating with references by ROW and COLUMN

Formula:

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