I am using ACF PRO V5.3.9.
Within the date picker field though, the Return Format is set to d-m-Y.
Also, when echoing out the sub_field within the repeater loop, the date outputs as d-m-Y.
I have now converted my $mon variable to the format suggested, Ymd, and it works. Awesome.
So mental note made: always use Ymd (yyyymmdd) as my base date formats!
Thanks very much James.