Difference between 2 dates in seconds

$timeFirst  = strtotime('2011-05-12 18:20:20');
$timeSecond = strtotime('2011-05-13 18:20:20');
$differenceInSeconds = $timeSecond - $timeFirst;

You will then be able to use the seconds to find minutes, hours, days, etc.

You don't need to change the php.ini file if you use date_default_timezone_set(). Just set it to the timezone you will be working in.

Something like this should go in a config file or on the page where you're working with dates (if it is only one page):

date_default_timezone_set('America/Los_Angeles');

First figure how many days there are between the two dates. Divide the number of days by 7 to get full weeks.

Now figure out if there's an extra week to be counted by finding taking the number of days modulus 7 to get any remaining days. If the first date plus remaining days falls in a different week, add an extra week on to the count.

void Main()
{

    var first = new DateTime(2018, 04, 13);
    var second = new DateTime(2018, 04, 16);

    Console.WriteLine(weekDiff(first, second));
}

public int weekDiff(DateTime d1, DateTime d2, DayOfWeek startOfWeek = DayOfWeek.Monday)
{
    var diff = d2.Subtract(d1);

    var weeks = (int)diff.Days / 7;

    // need to check if there's an extra week to count
    var remainingDays = diff.Days % 7;
    var cal = CultureInfo.InvariantCulture.Calendar;
    var d1WeekNo = cal.GetWeekOfYear(d1, CalendarWeekRule.FirstFullWeek, startOfWeek);
    var d1PlusRemainingWeekNo = cal.GetWeekOfYear(d1.AddDays(remainingDays), CalendarWeekRule.FirstFullWeek, startOfWeek);

    if (d1WeekNo != d1PlusRemainingWeekNo)
        weeks++;

    return weeks;
}