Get Interval Night and Day per Day between two dates - php

Question

I am creating an algorithm that must return two intervals between two dates:
$interval_day (between 6am and 9pm)
$interval_night (between 9pm and 6am)
Currently, I managed to have the overall interval between these two dates for the day. However, I now need a daytime interval and a nighttime interval for the day.
Here is the code I have done to get the interval per day between two dates:
foreach($period as $p => $v){
$day = $v->format('Y-m-d');
$interval = 0;
foreach($rotations as $rotation){
// Get the day of each dates
$date_start = explode(" ", $rotation['date_start'])[0];
$date_end = explode(" ", $rotation['date_end'])[0];
if($date_start == $date_end){
// += interval between the two dates
$interval += strtotime($rotation['date_end']) - strtotime($rotation['date_start']);
}else if($date_start == $day){
// += interval between the start date and midnight
$interval += strtotime($day." 23:59:59") - strtotime($rotation['date_end']);
}else if($date_end == $day){
// += interval between midnight and the end date
$interval += strtotime($rotation['date_end']) - strtotime($day." 00:00:00");
}
}
}
I hope it's clear for you
What I want now is to get 2 intervals instead of one :
1 interval for dates between 6 am and 9 pm as $interval_day
1 interval for dates between 9 pm and 6 am as $interval_night
For exemple:
rotation['date_start'] = 27/07/2018 21:00:00
rotation['date_end'] = 28/07/2018 02:00:00
then
27/07/2018 :
$interval_day = 00:00:00 and $interval_night = 03:00:00
28/07/2018 :
$interval_day = 00:00:00 and $interval_night = 02:00:00

Answer 1

For each day you have to compute 3 overlappings:
overlapping between the interval [date_start time_start,
date_end time_end] for the given day and the interval [date_start 06:00:00, date_end 21:00:00]
overlapping between the interval [date_start time_start, date_end time_end] for the given day and the interval [date_start 00:00:00, date_end 06:00:00]
overlapping between the interval [date_start time_start, date_end time_end) for the given day and the interval [date_start 21:00:00, date_end 23:59:59]
To do that, use the following formula:
overlap = MAX(0,end_1 - start_1 - MAX(0,end_1 - end_2) - MAX(0,start_2 - start_1))
The formula needs integers - i.e. UNIX timestamps.