Combine 2 Date + Time Columns?

This might be a piece of cake for some but it has me stumped.

I want to combine 'like shuffling a deck of cards' two date + time columns that overlap. The second column of dates+time of day must be placed into the first date+time column where it fits in chronological order. Example sheet attached which is very short compared to the many thousands of rows in the real world sheet. There is a corresponding data value associated with each date+time. The date+times are not necessarily in any consecutive order.

I wish to do this with formulas and not VBA if possible. Any ideas please.

Greg