when I use "modify" on my timestamp it gives a wrong output - php

Question

I want to block a someone who tried to login 4 times with a wrong password.
The problem is: When I use "modify" and change the timestamp with 15 minutes, I get -1 as output and after two minutes I get -2 as output. I tied for so long and I searched a lot on internet, but it still doesn't work.
How I want it to work:
In the database is the column: "falselog". If the username of the visiter is correct but the password is incorrect, falselog will be +1. When falselog is 4, the visiter will be banned for 15 minutes. So the visiter is able to try 4 times. After 15 minutes, the visiter can try again.
This is my object with all the code:
public function logUser($username, $password) {
// query id = 2
$sql2_1 = "SELECT
id,
password,
falselog,
lastlogin
FROM users
WHERE username ='".$username."' ";
$result2_1 = $this->con->query($sql2_1);
$fetch2_1 = mysqli_fetch_array($result2_1);
$count2_1 = $result2_1->num_rows;
$now = new DateTime('now');
$blockedSince = new DateTime($fetch2_1['lastlogin']);
$fout = $fetch2_1['falselog'];
$date_old = date ("Y-m-d H:i:s", strtotime("now"));
$block = date("i", $fetch2_1['lastlogin']) + 16;
$current = date("i", strtotime("now"));
$wait = $block - $current ;
$dbtime = date ("Y-m-d H:i:s", strtotime($date_old));
// This is the code that doesn't work
if ($fetch2_1['falselog']>=4 AND $blockedSince->modify('+15 minutes') > $now) {
$error[] = 'This account has been banned, try again about '.$wait.' minutes';
$decline = true;
$date_old = $fetch2_1['lastlogin'];
}
elseif (!preg_match("/^[A-Za-z0-9-]+$/", $username)) {
$error[] = 'De input bevat ongeldige tekens (alleen cijfers en letters toegestaan)';
}
elseif ($count2_1 === 0) {
$error[] = 'Wrong login data';
}
elseif ($fetch2_1['password']!=sha1($password)) {
$error[] = 'wrong password';
$fout = $fetch2_1['falselog']+1;
}
if ((count($error) == 0) OR ($fetch2_1['falselog']==4 AND $blockedSince->modify('+15 minutes') < $now)) {
$fout = 0;
}
$sql2_2 = "UPDATE users SET
falselog='".$fout."',
lastlogin='".$dbtime."'
WHERE username='".$username."' ";
if ($this->con->query($sql2_2) === TRUE) {
if (count($error) == 0) {
return false;
}
else {
return $error;
}
}
else {
echo "Error: " . $sql2_2 . "<br>" . $this->con->error;
return true;
}
}

Answer 1

You should check that here
$blockedSince = new DateTime($fetch2_1['lastlogin']);
you get the correct data for DateTime, try var_dump($blockedSince); after this line, and check that it has correct value inside;

Answer 2

try this
$blockedSince = mktime($fetch2_1['lastlogin']);