Date & Time - Drag down bug?

I am currently trying to plot two sets of data within the same time ranges. My time columns originally were organized as the date in one column and the time in the next. However, when plotting excel seemed to get confused when the time reset back to 12AM when the next day began so I decided I would amalgamate the two columns into one (i.e both date and time shown in one column).

I have a large set of data so I just wanted to extend the formula down so it auto-updated all my cells to the new format. Occasionally, for no apparent reason, it would skip a minute or double up on a minute. So at the end of the data range when it should have ended on 11:59 PM it ends 3 minutes too short or over time. See picture attached for an example.

My next issue would be that for some months there are missing chunks of time where I won't be able to simple drag the formula down to auto-populate. Is there a way I can reformat the time column to automatically detect the proper date by referencing the column to the left of it?