Day function reading month

Hi, I'm using a DAY function to read the day out of a column with the date in dd/mm/yyyy format.

However when the day is 12 or below the function reads the month instead, as you can see in the picture.

The column [days in between] reads the [ZAPPDATE] with a DAY function, but when the day value equals 12 or lower it instead reads the month value.

I have my dateformat set to dd/mm/yyyy on my computer so that shouldn't be the problem. Any ideas?

Thanks