Try this solutions:
$revenueMonth = Callback::where(
'created_at', '>=', Carbon::now()->subDays(30)->toDateTimeString()
);
You get all Callback for last 30 days.
$revenueMonth = Callback::where(
'created_at', '>=', Carbon::now()->firstOfMonth()->toDateTimeString()
);
Get for current month.
$revenueMonth = Callback::where(
'created_at', '>=', Carbon::now()->startOfMonth()->subMonth()->toDateString()
);
Get for start last month.
UPDATED
$revenueMonth = Callback::where(
'created_at', '>=', Carbon::now()->subMonth()->toDateTimeString()
);
This is what are you looking for :)
Hope it will help you:)