How to check if a date is in a given range?

Converting them to timestamps is the way to go alright, using strtotime, e.g.

$start_date = '2009-06-17';

$end_date = '2009-09-05';

$date_from_user = '2009-08-28';

check_in_range($start_date, $end_date, $date_from_user);


function check_in_range($start_date, $end_date, $date_from_user)
{
  // Convert to timestamp
  $start_ts = strtotime($start_date);
  $end_ts = strtotime($end_date);
  $user_ts = strtotime($date_from_user);

  // Check that user date is between start & end
  return (($user_ts >= $start_ts) && ($user_ts <= $end_ts));
}

Usage : $output = mergeRanges($input);

This method is originally designed to merge any kind on numeric ranges, timestamps included and supports any kind of overlaps. It takes an array of objects in input, containing "from" and "to" keys which is customizable.

/**
 * @param        $ranges
 * @param string $keyFrom
 * @param string $keyTo
 *
 * @return array
 */
function mergeRanges($ranges, $keyFrom = 'from', $keyTo = 'to')
{
    // Split from / to values.
    $arrayFrom = [];
    $arrayTo   = [];
    foreach ($ranges as $date)
    {
        $arrayFrom[] = $date->$keyFrom;
        $arrayTo[]   = $date->$keyTo;
    }

    // Sort ASC.
    natsort($arrayFrom);
    natsort($arrayTo);

    $ranges = [];
    // Iterate over start dates.
    foreach ($arrayFrom as $indexFrom => $from)
    {
        // Get previous entry.
        $previousEntry = end($ranges);
        // Find associated default "to" value to "from" one.
        $to = $arrayTo[$indexFrom];

        // If we have a previous entry and "to" is greater than
        // current "from" value.
        if (isset($previousEntry->to) && $from < $previousEntry->to + 1)
        {
            // Do nothing if this range is completely covered
            // by the previous one.
            if ($to > $previousEntry->to)
            {
                // We just change te "to" value of previous range,
                // so we don't create a new entry.
                $previousEntry->to = $to;
            }
        }
        else
        {
            // Create a new range entry.
            $ranges[] = (object) [
                $keyFrom => $from,
                $keyTo   => $to,
            ];
        }
    }

    return $ranges;
}

Example :

$input = [
    // One day.
    (object) [
        'title' => 'One day.',
        'from'  => 1560816000,
        'to'    => 1560902399,
    ],
    // Same day, inner period
    (object) [
        'title' => 'Same day, inner period',
        'from'  => 1560816000 + 1000,
        'to'    => 1560902399 - 1000,
    ],
    // Just before midnight
    (object) [
        'title' => 'Just before midnight',
        'from'  => 1560816000 - 1000,
        'to'    => 1560816000 + 1000,
    ],
    // Just after midnight
    (object) [
        'title' => 'Just after midnight',
        'from'  => 1560902399 - 1000,
        'to'    => 1560902399 + 1000,
    ],
    // Other period before
    (object) [
        'title' => 'Other period before',
        'from'  => 1560902399 - 100000,
        'to'    => 1560902399 - 100000 + 5000,
    ],
    // Other period after
    (object) [
        'title' => 'Other period after',
        'from'  => 1560816000 + 100000,
        'to'    => 1560902399 + 100000 + 5000,
    ],
];

Result :

Array
(
    [0] => Array
        (
            [from] => 2019-06-17 22:13:19
            [to] => 2019-06-17 23:36:39
        )

    [1] => Array
        (
            [from] => 2019-06-18 01:43:20
            [to] => 2019-06-19 02:16:39
        )

    [2] => Array
        (
            [from] => 2019-06-19 05:46:40
            [to] => 2019-06-20 07:09:59
        )

)

SELECT * FROM events 
  WHERE date1<='2012-01-18'
  AND date2>='2012-01-18'