<?php
 
// Datas vindas do banco de dados:
$dates = [
    ["2017-05-08 11:28:40", "2017-05-08 17:52:12"],
    ["2017-05-08 18:34:02", "2017-05-10 09:02:57"],
    ["2017-05-10 09:44:31", "2017-05-10 13:22:10"],
    ["2017-05-10 22:17:54", "2017-05-11 03:41:25"]
];
 
// Percorre os intervalos de datas:
foreach ($dates as $index => $date)
{
 
    // Cria um objeto DateTime para cada:
    $start = new DateTime($date[0]);
    $end   = new DateTime($date[1]);
 
    // Se a diferença entre as datas for superior a um dia:
    if ($end->diff($start)->format("%a") > 0)
    {
        // Lista de fatias do intervalo:
        $interval = [];
 
        // Enquanto a diferença for maior que um dia:
        while ($end->diff($start)->format("%a") > 0)
        {
            // Define a fatia do intervalo para o dia:
 
            $sliceStart = clone $start;
            $sliceEnd   = clone $start;
 
            $interval[] = [$sliceStart->format("Y-m-d H:i:s"), $sliceEnd->setTime(23, 59, 59)->format("Y-m-d H:i:s")];
 
            // Incrementa em um dia a data de início:
            $start->modify("+1 day")->setTime(0, 0, 0);
        }
 
        // Define o intervalo para o último dia:
        $interval[] = [$start->format("Y-m-d H:i:s"), $end->format("Y-m-d H:i:s")];
 
        // Substitui o intervalo original pela lista de fatias:
        $dates[$index] = $interval;
    }
}
 
print_r($dates);

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