Date not calculating correctly

Hi everyone,

I'm having a strange issue with a calculated date column and I was hoping you could help. I have a date, month, and year that I then consolidate into one column called "survey end date". The below example is showing the correct answer of 31/07/15:

Here's how the column is calculated:

The strange thing is that some answers are showing correctly, and some have the month and day switched around. In the below section of the results it's showing 04/09/19 instead of 09/04/19:

I can't think of what would cause this? They're from within the same data set so I can't see why some answers would be right and some wrong? The individual date, month, and year columns are formatted as whole numbers, and the Survey End Date column is formatted as date (dd/mm/yy).

Any help would be greatly appreciated!

Thanks