My Emacs version is GNU Emacs 28.0.50 (build 1, x86_64-apple-darwin19.6.0, NS appkit-1894.60 Version 10.15.6 (Build 19G73)) of 2020-07-29

I do the following expériment under Emacs CC

;; Set date format as Unix
dd U RET
;; Enter 1991-01-09T06:00 in Unix format, cf. date -d '1991-01-09T06:00+00' '+%s'
' <663400800> RET
;; Set format as ISO
dd YYYY-MM-DD HH:mm:SS RET

Then the display is <1991-01-10 06:00>, but I expected the 9th and not the 10th of January.

  Vincent.