Utility for flooring a date to a specified level

floorDate.js

function floorDate (floor /*, date, clone */) {

  var clone = (arguments[2] === true),
      date  = (typeof arguments[1] !== 'undefined') ? arguments[1] : new Date();
  
  if (clone || !(date instanceof Date)) {
    date = new Date(date);
  }

  switch(floor) {
    case 'year':   date.setMonth(0);
    case 'month':
    case 'week':   date.setDate(floor === 'week' ? date.getDate() - date.getDay() + floorDate.firstDayOfWeek: 1);
    case 'day':    date.setHours(0);
    case 'hour':   date.setMinutes(0);
    case 'minute': date.setSeconds(0);
    default:       date.setMilliseconds(0);
  }
  
  return date;
}

floorDate.firstDayOfWeek = 0;



new Date(2009, 1, 2, 3, 4, 5);
// Mon Feb 02 2009 03:04:05 GMT-0500 (EST)

floorDate('year', new Date(2009, 1, 2, 3, 4, 5));
// Thu Jan 01 2009 00:00:00 GMT-0500 (EST)

floorDate('month', new Date(2009, 1, 2, 3, 4, 5));
// Sun Feb 01 2009 00:00:00 GMT-0500 (EST)

floorDate('day', new Date(2009, 1, 2, 3, 4, 5));
// Mon Feb 02 2009 00:00:00 GMT-0500 (EST)

floorDate('hour', new Date(2009, 1, 2, 3, 4, 5));
// Mon Feb 02 2009 03:00:00 GMT-0500 (EST)

floorDate('minute', new Date(2009, 1, 2, 3, 4, 5));
// Mon Feb 02 2009 03:04:00 GMT-0500 (EST)

floorDate('second', new Date(2009, 1, 2, 3, 4, 5));
// Mon Feb 02 2009 03:04:05 GMT-0500 (EST)