How to count the number of unique date occurrences in a column

By far the easiest way is to use a Pivot Table. You get your answer in seconds. No formulas!

Click in the table with the dates, from the menu choose Organize > Create Pivot Table > On Current sheet and drag from the Fields box down to the Rows box as shown.

Click the circled i and switch from Year-Month to Value.

Select the cells in the Pivot Table with the distinct dates and look for COUNTA below.

You can drag that COUNTA token into a cell if you want.

SG

Here is another way. It uses formulas so it updates as soon as you enter new dates. Pivot tables are quick, easy, and powerful but the downside is they need to be refreshed after new data is entered.

Formula in C2 =IFERROR(1÷COUNTIF(B,B),0)

Fill down to complete the column

If you have no empty rows you don't need the IFERROR. It is there to catch division by 0 errors.

Formula in C1 =SUM(C)

This is number of distinct dates

If you put this formula somewhere else you can hide column C

It appears you are in a region where the decimal separator is a comma so the commas in formulas need to be semicolons.

=ALS.FOUT(1÷AANTAL.ALS(B;B);0)