Best practice: Using date as view key

My goal is to create a view which I can query documents with a range of date.
For example, let’s say I want to query documents created from Jan 01, 2015 to Feb 15, 2015.
I ended up with two ways of creating view key for such filter.

First: Composite Keys

var key = dateToArray(created_at);
// The key will look like...[2015,1,1] ... [2015,2,15]
emit(key, null);

Second: Single String Key

var date = dateToArray(created_at);
var year = date.getUTCFullYear();
var month = date.getUTCMonth();
var day = date.getUTCDate();
var key =
// and create something like... ["20150101"] ... ["20150215"]
emit(key, null);

Now, my question is that which way would be more efficient, with regard to how Couchbase indexing works, when querying documents with a range of date.

Any opinions and thoughts are welcome~:)