Yii2 datetime difference calculation between two dates and inserting the difference in db

There are 3 fields in task table-> expected_start_datetime,expected_end_datetime,time_allocated While creating a task expected start and end datetime is selected and saved in the records.

What I am trying to do is to find the difference between the two dates in hours and minutes and save the value inside the "time_allocated" while creating the task and later on the update or view page use/display the time allocated value from the records.

Trying something like this in the task controller action create

$diff = ((strtotime($model->expected_start_datetime) - strtotime($model->expected_end_datetime)) / (60 * 60 * 24));

        $model->time_allocated = $model->time_allocated + $diff;