Eloquent - Latest entry per user - laravel

Question

I'm trying to get the latest db entry for today for each user.
This is what I have, but it is returning the first entry, instead of the last.
$activity = UserJourney::where('datetime', '>', date('Y-m-d 00:00:00') )
->orderBy('id', 'desc')
->groupBy('user_id')
->get()
->keyBy('user_id')
;
Any ideas?
Thanks
EDIT:
This is the output:
[
{
"id": 1562,
"user_id": 10,
"datetime": "2015-02-05 11:03:41",
"url": "/",
"device": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:35.0) Gecko/20100101 Firefox/35.0",
"created_at": "2015-02-05 11:03:41",
"updated_at": "2015-02-05 11:03:41",
"deleted_at": null
},
{
"id": 1555,
"user_id": 148,
"datetime": "2015-02-05 09:20:56",
"url": "/dashboard/sales",
"device": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.94 Safari/537.36",
"created_at": "2015-02-05 09:20:56",
"updated_at": "2015-02-05 09:20:56",
"deleted_at": null
},
{
"id": 1553,
"user_id": 149,
"datetime": "2015-02-05 09:19:10",
"url": "/",
"device": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.94 Safari/537.36",
"created_at": "2015-02-05 09:19:10",
"updated_at": "2015-02-05 09:19:10",
"deleted_at": null
}
]