[Steve Dallas]

First, you can put two dates that are the same in column B, then highlight both of those dates. If you then click on the lower right corner of that selection and drag down to where the 23:30 time is, that will autofill all the dates so they are all the same.

Unfortunately, the autofill isn't intelligent enough to autofill for 6 months like that. You can fill blocks of one day at a time that way, which might speed things up.

That is the non-programming way to do this.

Then, there is using VBA to do this. I'm not sure what version of Excel you are using...and that might make things a little different.

Essentially, you can write a script that will fill a column with the dates, incrementing the date by one every 48 cells, until the date reaches the date you need to stop.