I'm not sure I understood your question... But here is one possible answer:
You can apply the formula COUNTIF to the next column like this:
The beginning of the interval will be fixed and the end will be increasing, while you push the formula down to new cells.
So, the fist time a date appears, the content of the cell will be "1". You can then format conditionally the cell for a specific format when it is "1".
If you don't like to see numbers, you can apply the function IF to get a more user friendly look to your file and adapt the conditional format accordingly :
Does this help?