You have to first create a list of unique dates.
If you have dates that are cumulatively ascending by 1 day, then try entering the first date in a new column, say C2, then copy that date down it should increment by 1 day, then you can apply formula.
Or you can select column A, and go to Data|Filter|Advanced Filter and select Unique Values Only, select a location like C2 and click Ok. This should give a unique list of dates, then apply formula to this list...
or you can create a Pivot Table like attached which does all the work for you.
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