To enter an array formula, type the formula in a cell then press and hold CTRL + SHIFT simultaneously, now press Enter once. Release all keys.
The formula bar now shows the formula with a beginning and ending curly bracket telling you that you entered the formula successfully. Don't enter the curly brackets yourself.
Explaining formula in cell E6
Step 1 - Find rows in cell range with date past
Dates in Excel are numbers formatted as dates. 1/1/1900 is 1. 1/1/2000 is 36526. If you enter a date in a cell Excel automatically converts and formats the date, however, if you enter a date in a formula you need to use the DATE function to convert the date to an Excel date.
The SMALL function makes sure that a new value is returned in each row, the second argument $F$6:F6 is expanding when you copy the cell and paste to cells below. This adds 1 to the second argument for each cell below.
The second argument in the SMALL function contains the COUNTIF function, it is designed to count how many times the adjacent value has been displayed before. The first cell reference in the COUNTIF function expands as the cell is copied to cells below. This lets the formula keep track of previous values, we want it to return the correct date value even if there are duplicates in column B.