Check if two times overlap

Question

I want to see if a time I read from a db overlaps with a time provided by a user.

My database looks like this:

-----------------------------------------------
|organiser|meeting_start|meeting_end|boardroom|
-----------------------------------------------
| John Doe| 1340193600  | 1340195400| big     |
-----------------------------------------------

My code looks like this:

date_default_timezone_set('Africa/Johannesburg');
$from = strtotime($_GET['meeting_date'] . ' ' . $_GET['meeting_start']);
$to = strtotime($_GET['meeting_date'] . ' ' . $_GET['meeting_end']);
$another_meeting = false;
$meeting_date = strtotime($_GET['meeting_date']);
$meeting_next = $meeting_date + 86400;

$result = mysql_query("SELECT meeting_start, meeting_end FROM admin_boardroom_booking WHERE boardroom = '" . $_GET['boardroom'] . "' AND meeting_start >= '" . $meeting_date . "' AND meeting_end < '" . $meeting_next . "'")or die(mysql_error());
while($row = mysql_fetch_array($result)) {
    $from_compare = $row['meeting_start'];
    $to_compare = $row['meeting_end'];

    $intersect = min($to, $to_compare) - max($from, $from_compare);
    if ( $intersect < 0 )
        $intersect = 0;

    $overlap = $intersect / 3600;
    if ( $overlap <= 0 ) {
        $another_meeting = true;
        break;
    }
}

if ($another_meeting)
    echo 'ERROR';

If I type two overlapping times on purpose, it doesn't echo error. What am I doing wrong?

Gobbledygook

Your code is vulnerable to SQL injections. Please read this: https://mcmap.net/q/119307/-what-is-sql-injection-duplicate and switch over to parameterised queries. The old mysql_ functions are soon-to-be-deprecated, you should use MySQLi instead. – Kerenkeresan

Tanks for the help. That did however not solve my problem. – Gobbledygook

@DarkRanger : can you type echo $result before while loop and let me know what you get? – Kirven

Answer 1

Two time periods P1 and P2 overlaps if, and only if, at least one of these conditions hold:

1. P1 starts between the start and end of P2 (P2.from <= P1.from <= P2.to)
2. P2 starts between the start and end of P1 (P1.from <= P2.from <= P1.to)

This will catch partly overlapping periods as well as periods where one completely covers the other. One of the periods must always start (or end) inside the other if they are overlapping.

So $another_meeting would be defined by:

$another_meeting = ($from >= $from_compare && $from <= $to_compare) ||
                   ($from_compare >= $from && $from_compare <= $to);

You may want to change the borderline cases to strict < checks if one event can start at the exact same time as another ends.

Raceme

This seems to be working perfectly. I've changed it a bit to allow exact same times (I'm working with engineers, and specifying 14:01 instead of just 14:00 seems to be a major brainflaw). Baie Dankie (Thank you very much in Afrikaans) – Gobbledygook

Answer 2

Was just doing something similar.... but just with times....

$startTime = strtotime("7:00");
$endTime   = strtotime("10:30");

$chkStartTime = strtotime("10:00");
$chkEndTime   = strtotime("12:10");

if($chkStartTime > $startTime && $chkEndTime < $endTime)
{
    // Check time is in between start and end time
    echo "1 Time is in between start and end time";
}
elseif(($chkStartTime > $startTime && $chkStartTime < $endTime) || ($chkEndTime > $startTime && $chkEndTime < $endTime))
{
    // Check start or end time is in between start and end time
    echo "2 ChK start or end Time is in between start and end time";
}
elseif($chkStartTime==$startTime || $chkEndTime==$endTime)
{
    // Check start or end time is at the border of start and end time
    echo "3 ChK start or end Time is at the border of start and end time";
}
elseif($startTime > $chkStartTime && $endTime < $chkEndTime)
{
    // start and end time is in between  the check start and end time.
    echo "4 start and end Time is overlapping  chk start and end time";
}

Kennedy

Answer 3

I'd probably solve it with something like this:

function avaliable($start, $end) {
  // checks if there's a meeting between start or end
  $q = "SELECT * FROM admin_boardroom_booking "
    . "WHERE NOT (meeting_start BETWEEN '$end' AND '$start' "
    . "OR meeting_end BETWEEN '$end' AND '$start')";
  $result = mysql_query($q);

  // returns true on no conflicts and false elseway
  return mysql_num_rows($result) === 0;
}

Plumbism

Might work, but then I would also need to checkk if the meeting doesn't overlap another one completely. Example: Meeting 1 starts. Meeting 2 starts, meeting 2 ends, meeting 1 ends. Looking at your code, it wont return false in this case... – Gobbledygook

Your conditions are unfortunately wrong. What happens if $start is before meeting_start and $end is after meeting_end? The parentheses will evaluate to false and you negate it into true! It's also a rather strange approach to fetch all non-conflicting meetings when you really just want to know if there are conflicts or not. Try to match conflicting meetings instead and use COUNT(*) and LIMIT 1 to speed it up a bit. – Angeloangelology

It seems like I'm a bit tired, I'm going to rewrite it >___< – Plumbism

Answer 4

Emil Vikström's answer is correct, but there is a scenario needed to be considered.
Like, One of the time ranges is a subset of another time range.
So, assume that P1{start_time, end_time} and P2{start_time, end_time} will overlap when any of the following is true.

• P1.start_time <= P2.start_time <= P1.end_time
• P1.start_time <= P2.end_time <= P1.end_time
• P2.start_time <= P1.start_time <= P1.end_time <= P2.end_time

Just assuming that the times are sorted in ascending order. Example below:

|-----------------------------------|
|  Start time  |   End time  | Name |
|-----------------------------------|
|    10:00     |    14:00    |  P1  |
|-----------------------------------|
|    12:00     |    16:00    |  P2  |
|-----------------------------------|
|    08:00     |    12:00    |  P3  |
|-----------------------------------|
|    07:00     |    16:00    |  P4  |
|-----------------------------------|

If you consider P1 as the base time and you want to check P2, P3, P4 against it.

1. P1.start_time <= P2.start_time <= P1.end_time true
2. P1.start_time <= P3.end_time <= P1.end_time true
3. P4.start_time <= P1.start_time <= P1.end_time <= P4.end_time true

This is how you can check if any of the time is overlapping another one or not.

Fedora

My answer already solves this. If P1 is a full subset of P2, then my first condition holds true: P1 will start within P2. – Angeloangelology

Answer 5

In case when $to is always later than $from we can use this more shorter solutions

$another_meeting = !($from > $to_compare || $from_compare > $to);

Learn

Answer 6

My solution that deals with the edge cases and is quite compact:

    private function meetingOverlap($meeting1, $meeting2){
        if ($meeting1['start']>$meeting2['start'] && $meeting1['start']<$meeting2['end'] ) return true;
        if ($meeting2['start']>$meeting1['start'] && $meeting2['start']<$meeting1['end'] ) return true;
        if ($meeting2['start']==$meeting2['start'] && $meeting1['end']==$meeting2['end'] ) return true;
        return false;
    }

Danieladaniele