Check for british summer time (BST) in JavaScript

is_british_summertime.js

function lastSunday(month, year) {
  var d = new Date();
  var lastDayOfMonth = new Date(Date.UTC(year || d.getFullYear(), month+1, 0));
  var day = lastDayOfMonth.getDay();
  return new Date(Date.UTC(lastDayOfMonth.getFullYear(), lastDayOfMonth.getMonth(), lastDayOfMonth.getDate() - day));
}

function isBST(date) {
  var d = date || new Date();
  var starts = lastSunday(2, d.getFullYear());
  starts.setHours(1);
  var ends = lastSunday(9, d.getFullYear());
  ends.setHours(1);
  return d.getTime() >= starts.getTime() && d.getTime() < ends.getTime();
}

Made a new version that seems to work and is a bit simpler: https://gist.github.com/liam-jones-lucout/1c8b19ad6fcaf4103bb4b6af97877cd7