Given a JS date object, get an ISO 8601 string for the beginning of that day in a target timezone.

beginning-of-day.js

var beginningOfDayWithOffset = function (targetOffset) {
  var pad = function (n) { return (n.toString().length === 2 ? n.toString() : "0" + n) }

  var sign = "-";
  var offsetHours, offsetMinutes, offsetString;

  // That's right, JavaScript's timezone offset has the opposite sign of the
  // offset in ISO 8601...
  if (targetOffset <= 0) {
    sign = "+";
    targetOffset = -targetOffset;
  }

  offsetHours = parseInt(targetOffset / 60);
  offsetMinutes = targetOffset % 60;
  offsetString = sign + pad(offsetHours) + pad(offsetMinutes);

  return function (date) {
    var localOffset = date.getTimezoneOffset();
    var offsetFromTarget = targetOffset - localOffset;
    var bod = new Date(date);

    // This will have the effect of adding or subtracting hours to make the
    // "local" time match the local time in the target time zone, crossing day
    // boundaries if necessary.
    bod.setMinutes(bod.getMinutes() + offsetFromTarget);

    return bod.getFullYear() + "-" +
      pad(bod.getMonth() + 1) + "-" +
      pad(bod.getDate()) + "T00:00:00" +
      offsetString;
  }
}

// For example, EST (UTC-5):
var beginningOfDayEST = beginningOfDayWithOffset(300);