PHP Academy · Lesson

Date Arithmetic with DateInterval

Add and subtract time periods using DateInterval and modify().

Date Arithmetic with DateInterval is a free PHP Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the PHP Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

DateInterval Overview

DateInterval represents a duration of time — days, months, hours, etc. It's the PHP OO way to add or subtract time from DateTime objects.

Creating DateInterval

Create a DateInterval using ISO 8601 duration notation:

<?php
// P = period, T = time separator
$oneYear  = new DateInterval('P1Y');    // 1 year
$sixMonths = new DateInterval('P6M');   // 6 months
$tenDays  = new DateInterval('P10D');   // 10 days
$twoHours = new DateInterval('PT2H');   // 2 hours (T before time)
$complex  = new DateInterval('P1Y2M3DT4H5M6S');

echo $oneYear->y;    // 1
echo $tenDays->d;    // 10

add() and sub()

Add or subtract a DateInterval from a DateTime:

<?php
$dt = new DateTime('2024-01-01');

$dt->add(new DateInterval('P1Y'));   // +1 year
echo $dt->format('Y-m-d');  // 2025-01-01

$dt->sub(new DateInterval('P3M'));   // -3 months
echo $dt->format('Y-m-d');  // 2024-10-01

diff() Between Two Dates

diff() returns a DateInterval representing the difference between two dates:

<?php
$start = new DateTime('2024-01-01');
$end   = new DateTime('2024-06-15');

$diff = $start->diff($end);

echo $diff->y;  // years
echo $diff->m;  // months
echo $diff->d;  // days
echo $diff->h;  // hours
echo $diff->days; // total days: 166

Human-Readable Differences

Format a DateInterval for display:

<?php
$birthdate = new DateTime('1990-05-15');
$today     = new DateTime();
$age       = $birthdate->diff($today);

echo $age->y . ' years, ' . $age->m . ' months, ' . $age->d . ' days';

// Check if date is in the past
if ($diff->invert === 1) {
    echo 'Date was in the past';
}

DateInterval::createFromDateString()

Create a DateInterval from a natural language string:

<?php
$interval = DateInterval::createFromDateString('1 year 2 months 3 days');

$dt = new DateTime('2024-01-01');
$dt->add($interval);
echo $dt->format('Y-m-d');  // 2025-03-04

// Other examples:
DateInterval::createFromDateString('next monday');
DateInterval::createFromDateString('6 weeks');

DatePeriod — Iterate Over Ranges

DatePeriod lets you iterate over a date range at regular intervals:

<?php
$start    = new DateTime('2024-01-01');
$interval = new DateInterval('P1M');  // monthly
$end      = new DateTime('2024-06-01');

$period = new DatePeriod($start, $interval, $end);

foreach ($period as $date) {
    echo $date->format('Y-m') . PHP_EOL;
}
// 2024-01, 2024-02, 2024-03, 2024-04, 2024-05

Using DatePeriod for Weekly Reports

Generate weekly date checkpoints:

<?php
$start    = new DateTime('2024-01-01');
$interval = new DateInterval('P7D');  // weekly
$end      = new DateTime('2024-02-01');

$weeks = new DatePeriod($start, $interval, $end);
foreach ($weeks as $week) {
    echo 'Week of: ' . $week->format('Y-m-d') . PHP_EOL;
}

Comparing Intervals

Compare the magnitude of two DateIntervals by converting to total days:

<?php
$a = new DateTime('2024-01-01');
$b = new DateTime('2024-06-01');
$c = new DateTime('2025-01-01');

$diff1 = $a->diff($b)->days;  // 152
$diff2 = $a->diff($c)->days;  // 366

echo ($diff1 < $diff2) ? 'First interval shorter' : 'First interval longer';
// First interval shorter

Business Days Calculation

Calculate the number of business days between two dates:

<?php
function businessDays(DateTime $from, DateTime $to): int {
    $count = 0;
    $cur   = clone $from;
    while ($cur < $to) {
        $dow = (int) $cur->format('N'); // 1=Mon ... 7=Sun
        if ($dow < 6) $count++;
        $cur->modify('+1 day');
    }
    return $count;
}

$from = new DateTime('2024-05-27');
$to   = new DateTime('2024-06-07');
echo businessDays($from, $to) . ' business days';

Expiry Date Pattern

Common subscription/token expiry pattern using DateInterval:

<?php
function expiresAt(string $duration = 'P30D'): string {
    $dt = new DateTimeImmutable();
    return $dt->add(new DateInterval($duration))->format('Y-m-d H:i:s');
}

echo expiresAt('P30D');   // 30-day trial expiry
echo expiresAt('P1Y');    // 1-year subscription expiry
echo expiresAt('PT1H');   // 1-hour token expiry

Quick Check

What does $dateA->diff($dateB)->days return?

Recap: DateInterval

Key points:

new DateInterval('P1Y2M3DT4H') — ISO 8601 duration
add()/sub() to modify DateTime
diff() returns DateInterval between two dates
DatePeriod iterates over a date range
Use ->days for total day count from diff()
Check ->invert to know if interval is negative

Frequently asked questions

Is the “Date Arithmetic with DateInterval” lesson free?

Yes — the full text of “Date Arithmetic with DateInterval” is free to read here on the web, and the PHP Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the PHP Academy course, upgrade to CoddyKit PRO.

What will I learn in “Date Arithmetic with DateInterval”?

Add and subtract time periods using DateInterval and modify(). You practise PHP Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start PHP Academy?

No prior experience is required. PHP Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Date Arithmetic with DateInterval” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this PHP Academy lesson?

Yes. Every PHP Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to PHP Academy