The cookie should store
year-month-day ( TDateTime order) and
UTC, since that should be programmer native and
only convert for display purposes based on locale settings.
Anything other would be a common design flaw. Storing dates is not complex, but sometimes even advanced programmers fall into that trap.
So if that is the case report a bug.
BUT to my knowledge, fcl-web is correct, so likely you made a programming error where you assume your own locale and that is wrong.
See
https://en.wikipedia.org/wiki/Coordinated_Universal_TimeAny software that does not adhere to UTC for date time storage would fail at different time zones.
(If I wasn't officially mad it would drive me nuts)