经过我们的交谈，我想我已经了解了问题的全貌，所以让我们看看是否可以一起解决这个问题：
我不确定json从何而来，所以我们先把它赋给一个变量：

$jsonString = '
{
  "2022 - 12 - 20": [
    {
      "dayNumber": 2,
      "startHours": "08:00",
      "endHours": "10:00"
    },
    {
      "dayNumber": 3,
      "startHours": "11:00",
      "endHours": "16:00"
    }
  ],
  "2022 - 12 - 28": [
    {
      "dayNumber": 2,
      "startHours": "08:00",
      "endHours": "10:00"
    },
    {
      "dayNumber": 3,
      "startHours": "11:00",
      "endHours": "16:00"
    }
  ],
  "2023 - 01 - 01": [
    {
      "dayNumber": 5,
      "startHours": "05:00",
      "endHours": "10:00"
    },
    {
      "dayNumber": 7,
      "startHours": "19:00",
      "endHours": "22:00"
    }
  ]
}
';

然后，我们可以这样运行代码：

//I'm assuming that the json only contains availabilities for 1 user
//This creates an associated array out of your json
$assocArray = json_decode($jsonString, true);
$totalAvailabilityInMinutes = 0;
foreach($assocArray as $updatedAvailabilityAt => $availabilityArray) {
    $availabilityOfDayInMinutes = 0;
    foreach($availabilityArray as $availability) {
        $explodedStart = explode(':', $availability['startHours']);
        $explodedEnd = explode(':', $availability['endHours']);
        //Perform calculation from my comment
        $availabilityOfDayInMinutes = ($explodedEnd[0] * 60 + $explodedEnd[1]) - ($explodedStart[0] * 60 + $explodedStart[1]);
        dump("Availability of day {$availability['dayNumber']}: $availabilityOfDayInMinutes");
        $totalAvailabilityInMinutes += $availabilityOfDayInMinutes;
    }
}
dump($totalAvailabilityInMinutes);

请注意，我使用了两个变量来存储分钟数，一个是每天的，另一个是累积天数，您可以选择哪个变量最适合您。
我已经在本地测试了代码，它应该工作，并提供正确的数字;）

编辑：

因此，既然您已经有了一个数组，而不是一个JSON，那么您可以跳过这一步：

$assocArray = json_decode($jsonString, true);