This should generate a series of 1-day timeslices for the previous 30 days (including current day).

Timeslice Generator

var start = new Date();
start.setHours('0');
start.setMinutes('0');
start.setSeconds('0');
start.setMilliseconds('0');

var end = new Date( start.getTime() );
end.setDate( end.getDate() + 1 );

var queries = [];
for (var d = 30; d > 0; d--) {

  start.setDate( start.getDate() - d );
  end.setDate( start.getDate() + 1 );

  queries.push({
      $gte: start
    , $lt: end
  });
}