Zend_Date: How to get the date of an upcoming day?

Question

I want to get the date of the closest Monday in the future (i.e. not in the past).

So if today is Tuesday (Dec 1, 2009) I want to get the date of Monday (Dec 7, 2009).

How can I do this with Zend_Date?

Solution:

Let's say today is Tuesday and we wanted to get the upcoming Monday. Monday is 6 days into the future. So, we would add 6 days to get monday's date.

//like so:
$tuesday = Zend_Date::now();
$nextMonday = $tuesday->addDay(6);

To do this dynamically, we will need to determine which day of the week it is today:

$today = Zend_Date::now();
$dayNumber = $today->get(Zend_Date::WEEKDAY_DIGIT);
//dayNumber will now be equal to the numeric day of the week (0-6)
//example:
$weekdays = array(
    0 => 'sunday',
    1 => 'monday',
    2 => 'tuesday' //etc...
);

To determine how many days we need to add to get the desired future day, we do the following:

$daysToAdd = ( $dayWanted - $todayDayNumber + 7 );
#      $dayWanted = monday(1)
# $todayDayNumber = tuesday(2)
#               7 = number of days in a week (we don't want a negative number)
#       1 - 2 + 7 = 6 days into the future
$nextMonday = $today->addDay($daysToAdd);

Let's say the day we want is wednesday (tomorrow), one day into the future. Our previous solution won't work:

$daysToAdd = ( $dayWanted - $todayDayNumber + 7 );
#      $dayWanted = wednesday(3)
# $todayDayNumber = tuesday(2)
#               7 = number of days in a week
#       3 - 2 + 7 = 8 days into the future (not 1)

We can solve this problem by adding the modulus operator (percent sign) to our formula to get the remainder of a division operation.

$daysToAdd = ( $dayWanted - $todayDayNumber + 7 ) % 7;
# (3 - 2 + 7) % 7
# $daysToAdd == 1 (remainder of 8 divided by 7)
$tomorrow = $today->addDay($daysToAdd);

Now our formula will work as expected...with the exception of one thing. If today is tuesday, and I want to get next tuesday, our formula will return today instead of a week from today:

$daysToAdd = ( $dayWanted - $todayDayNumber + 7 ) % 7;
# (2 - 2 + 7) % 7 == 0
# 7 goes into 7 evenly with no remainder

We will have to add a check to make sure it is not equal to zero.

if ($daysToAdd == 0) {
    //give me the date a week from today, not today's date
    $daysToAdd = 7;
}

Final Solution:

public function outputDate()
{
    $monday = $this->getDateOfNext('monday');
    echo 'today: ' . Zend_Date::now()->toString(Zend_Date::RFC_850) . "<br>";
    echo "monday: " . $monday->toString(Zend_Date::RFC_850);
}

private function getDateOfNext($dayWanted)
{
    $weekdays = array('sunday', 'monday', 'tuesday', 'wednesday', 'thursday', 'friday', 'saturday');
    if (!in_array($dayWanted, $weekdays)) {
        throw new Exception("'$dayWanted' not found in array of possible weekdays");
    }
    $weekdays = array_flip($weekdays);
    $date = Zend_Date::now();
    $today = $date->get(Zend_Date::WEEKDAY_DIGIT);
    $daysToAdd = ( $weekdays[$dayWanted] - $today + 7 ) % 7;
    if ($daysToAdd == 0) {
        //give me the date a week from today, not today's date
        $daysToAdd = 7;
    }
    $date->addDay($daysToAdd);
    return $date;
}

Impediment

Answer 1

Here's the logic, laid out:

$days_per_week = 7;
$weekdays = array_flip(array('sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat'));

$day_wanted = 'mon';
$days_forward =
  ( $weekdays[$day_wanted] - $date->get(Zend_Date::WEEKDAY_DIGIT) + $days_per_week )
  % $days_per_week;

$date->addDay($days_forward);

That works nicely for any $day_wanted.

Wetmore

what does the ( ) % $days_per_week; do? Specifically the percent sign – Impediment

also...what if today is tuesday, and you want to get the next tuesday? that should be 7 days forward, instead of 0. How can this be achieved? – Impediment

The % (mod function) keeps $days_forward between 0 and 6 inclusive. If you want to jump ahead 7 days, then I'd do it after the main calculation this way: if ($days_forward == 0) $days_forward = $days_per_week. – Wetmore