The timezone yes, daylight savings no. Think about it. If it didn't then every program anyone wrote that created Date objects would be an hour out for 6 months of the year.
Otherwise why do
var bst = new Date(2014,9,25,10,00,00);
var gmt = new Date(2014,9,27,10,00,00);
Give two different times when converted to UTC?
And converting the other way, why do
var bst = new Date(Date.UTC(2014,9,25,9,00,00));
var gmt = new Date(Date.UTC(2014,9,27,10,00,00));
Both give you 10am, the first with a BST time stamp and the second with a GMT one?