Round a timestamp to the nearest date

Question

I need to group a bunch of items in my web app by date created.

Each item has an exact timestamp, e.g. 1417628530199. I\'m using Moment.js and its \"tim

Answer 1

function roundDownDate(date) {
  if (typeof date !== "object" || !date.getUTCMilliseconds) {
      throw Error("Arg must be a Date object.");
  }
  var offsetMs = date.getTimezoneOffset() * 60 * 1000,
      oneDayMs = 24 * 60 * 60 * 1000;
  return new Date(Math.floor((date.getTime() - offsetMs) / oneDayMs) * oneDayMs + offsetMs);
};

This should work and is pretty fast.