Pivot table sees extracted day, month, year as numbers

Hi - I'm not the strongest spreadsheet user by a long chalk. I got good guidance here when I first stepped towards pivot tables. Now I'm a bit stuck.

My data for analysis started with a column: DD/MM/YYYY. I chose to add three columns referenced to this original column for DD, MM & YYYY (=DAY(D2), =MONTH(D2), =YEAR(D2).

Whenever I bring these columns' data to my pivot table the fields for these three new columns only offer a sum return - it's seeing the day, month and year as numbers. The original date column doesn't do this - it recognises the data as date.

I've tried to rectify by formatting the data as dates, but this doesn't stick - when I click back into a cell I've just formatted as date it reports as automatic with the option available to give it decimal output, +/- and thousands separator.

I've tried the format the data as text (I'm not sure if that's a good idea or not) and the same thing happened: the data retain their automatic formatting and when chosen as fields and added to columns, rows or values in the Organise pane offer themselves as summative.

Can anybody point me in a good direction from here, please?