Formula & Merge Help

[_RAS_]

Oh, I hope someone can help save me hours of work… Example attached below.

Column A is a sequential list of dates & times. B4:B7 is populated with values which are not to change as they are assigned to corresponding time in column A. The next set of data is in column C8:C11, D12:D15 and so on. The next time column B is populated is at B28:B31 & then C32:C35. Cell B8 is merged down to B27 & Cell C12 merged to C31. This goes on for many columns and rows.

I am looking to fill the gap (merged cells) of time within each column. I can type in a subtraction formula for each period: for column B, I use A28minusA7 & in column C, A32minusA11.

Is there a formula to keep the constant column A & then row after selected cell minus row before selected cell, assuming the selected cell is the merged time gap?

Also, is there a short-cut to merging all blank cells of my columns? I’m a novice & highlight everything then use the merge button. There’s gotta be an easier way… Thanks!
Using Excel 2007