When dates are held in a collection, reading of those dates shows BST daylight saving applied to those dates, silently, giving the impression that dates entered are incorrect.
1) Create a collection containing a Date Time column and populate with dates and time throughout the year.
getTimezoneOffset() is always set to 0 so there is no way, that I can find, to detect that BST daylight saving has been applied to particular rows.
The work around is to write some ugly JS to manually detect if the date in question is within 25 March to 28 October and reset those dates' times to the values entered.
How Wix can improve matters.
Do not manipulate dates silently when they are extracted from a Collection.