DB requery with php for envoriment monitoring - php

Question

I am new here and this is my first post
I need help for a college project.
We are programming a raspberry pie so that is measures temperature and air humidity and visualize the data on an html page.
But we have a problem to make the visualize right.
We take the data from myadminphp and show it graphically, but the time stamp is not steady it change the time to time at the moment minus 3 hours but we want to have it fixed every 3 hours.
Php code:
$lastDatetime = $db->query("select date_time
from measurements
where id=(
select max(id)
from measurements
)")
->fetch(PDO::FETCH_ASSOC)["date_time"];
$thisDay = $lastDatetime;
$dataLastSevenDays = array();
for ($i = 0; $i < 56; $i++) {
$maxTemperatureThisDay = $db->query("select value
from measurements
where date_time like '" . substr($thisDay, 0, 13) . "%'
and channel = 1
limit 1"
)
->fetch(PDO::FETCH_ASSOC)["value"];
//channel 1 Temp
$maxBrightnessThisDayThisDay =
$db->query("select value
from measurements
where date_time like '" . substr($thisDay, 0, 13) . "%'
and channel = 2
limit 1")
->fetch(PDO::FETCH_ASSOC)["value"]; //channel 2 Licht
$maxHumidityThisDay =
$db->query("select value
from measurements
where date_time like '" . substr($thisDay, 0, 13) . "%'
and channel = 4
limit 1")
->fetch(PDO::FETCH_ASSOC)["value"]; //channel 4 Luftfeuchtigkeit
$thisDay = date_sub(new DateTime($thisDay), new DateInterval('PT10800S'))->format('Y-m-d H:i:s');
$dataLastSevenDays[$i] = array($thisDay, $maxTemperatureThisDay, $maxBrightnessThisDayThisDay, $maxHumidityThisDay);
}
the html code:
<script>
var ctx = document.getElementById('temperature').getContext('2d');
var chart = new Chart(ctx, {
type: 'line',
data: {
labels: [
<?php
for ($i = 56; $i > 0; $i--) {
echo '"' . $dataLastSevenDays[$i][0] . '",';
}
?>
],
datasets: [{
label: "gemessene Temperatur",
borderColor: 'rgb(16, 119, 237)',
data: [
<?php
for ($i = 56; $i > 0; $i--) {
echo '"' . $dataLastSevenDays[$i][1] . '",';
}
?>
]}]
},
options: {}
});
</script>