PHP: Math and Date Functions

Q: date() or DateTime — which should I use?

Use date() for simple one-line formatting. Reach for DateTime when you need to calculate date differences, iterate over date ranges, modify dates, or compare them.

Q: Why is my time off by 8 hours?

Timezone. Call date_default_timezone_set("Asia/Shanghai") in your script (or set date.timezone = Asia/Shanghai in your php.ini ).

Q: What does strtotime("2026/02/29") return?

February 29 doesn't exist in 2026. strtotime auto-corrects and returns 2026-03-01 . DateTime handles this the same way.

Calculating discounts, building reports, displaying "posted 3 days ago," generating order numbers — math and date operations are essential to every web application.

1. Common Math Functions

PHP

<?php
// Basic operations
echo abs(-5);             // 5 (absolute value)
echo ceil(3.14);          // 4 (round up)
echo floor(3.99);         // 3 (round down)
echo round(3.5);          // 4 (rounds .5 to nearest even)
echo round(3.14159, 2);   // 3.14 (2 decimal places)

// Min and max
echo max(3, 7, 2, 9);     // 9 (maximum)
echo min(3, 7, 2, 9);     // 2 (minimum)
echo max([5, 3, 9, 1]);   // 9 (arrays work too)

// Powers and roots
echo pow(2, 3);           // 8 (2 to the power of 3)
echo sqrt(16);            // 4 (square root)

// Random numbers
echo rand(1, 100);        // Random integer between 1 and 100
echo mt_rand(1, 1000);    // High-quality random (recommended)
echo random_int(1, 10);   // Cryptographically secure random (PHP 7+)
?>

💡 Tip: For everyday randomness, use mt_rand() — it's about 4× faster and produces better randomness than rand(). For security-sensitive values like passwords and tokens, use random_int() instead, which is cryptographically secure.

2. The date() Function

date() is PHP's workhorse for formatting dates and times via a format string:

PHP

<?php
// Current date and time
echo date("Y-m-d");             // 2026-06-29
echo date("l, F j, Y");         // Monday, June 29, 2026
echo date("H:i:s");             // 14:30:45
echo date("Y-m-d H:i:s");       // 2026-06-29 14:30:45

// Day of week
echo date("l");                 // Monday (full weekday name)
echo date("w");                 // 1 (numeric day, 0 = Sunday)
?>

(1) Common Format Characters

Character	Meaning	Example
`Y`	Four-digit year	2026
`y`	Two-digit year	26
`m`	Month (zero-padded)	06
`n`	Month (no padding)	6
`d`	Day (zero-padded)	09
`j`	Day (no padding)	9
`H`	24-hour (zero-padded)	14
`i`	Minutes (zero-padded)	30
`s`	Seconds (zero-padded)	45

▶ サンプル: Timestamp Conversions

PHP

<?php
// time(): current Unix timestamp (seconds since January 1, 1970)
$now = time();
echo $now;  // e.g. 1751101234

// date() accepts a timestamp as its second argument
echo date("Y-m-d H:i:s", $now);

// strtotime(): human-readable dates → timestamps
$ts = strtotime("2026-01-01 00:00:00");
echo date("F j, Y", $ts);  // January 1, 2026

// strtotime is surprisingly smart
echo date("Y-m-d", strtotime("+1 week"));     // One week from now
echo date("Y-m-d", strtotime("next Monday")); // Next Monday
echo date("Y-m-d", strtotime("last day of this month")); // Last day of this month
echo date("Y-m-d", strtotime("+3 months"));   // Three months from now
?>

▶ 試してみよう

💡 Tip: strtotime() understands natural-language phrases like +1 week, next Monday, and first day of next month. That said, don't lean on it for complex date math — use the DateTime class instead.

3. The DateTime Class

date() and strtotime() cover the simple cases. For anything more complex, reach for the DateTime class — it's object-oriented and far more powerful:

PHP

<?php
// Creating DateTime objects
$now = new DateTime();
$specific = new DateTime("2026-06-01 09:00:00");
$fromFormat = DateTime::createFromFormat("d/m/Y", "15/08/2026");

// Formatting output
echo $now->format("Y-m-d H:i:s");

// Adding and subtracting dates
$future = new DateTime();
$future->modify("+2 weeks");
$future->modify("+3 days");
echo $future->format("Y-m-d");  // Today + 17 days

// Calculating differences
$birthday = new DateTime("2000-01-15");
$today = new DateTime();
$diff = $birthday->diff($today);
echo "You have been alive for {$diff->days} days, {$diff->y} years";
// diff() returns a DateInterval object with y/m/d/h/i/s/days properties
?>

▶ サンプル: Date Comparison and Ranges

PHP

<?php
$start = new DateTime("2026-06-01");
$end   = new DateTime("2026-06-30");

// Comparing dates
if ($start < $end) {
    echo "Start date is before end date";
}

// Iterating over a date range
$period = new DateInterval("P7D");     // 7-day interval
$dates = new DatePeriod($start, $period, $end);
foreach ($dates as $date) {
    echo $date->format("m-d") . " ";
}
// Output: 06-01 06-08 06-15 06-22 06-29
?>

▶ 試してみよう

4. Timezone Configuration

PHP

<?php
// Set the timezone (do this once at the top of your script)
date_default_timezone_set("Asia/Shanghai");

echo date("Y-m-d H:i:s");  // Now shows Shanghai time

// Check the current timezone
echo date_default_timezone_get();  // Asia/Shanghai

// Common timezones:
// Asia/Shanghai    - China Standard Time (UTC+8)
// Asia/Tokyo       - Japan Standard Time (UTC+9)
// Europe/London    - Greenwich Mean Time / British Summer Time
// America/New_York - Eastern Time (UTC-5/UTC-4)
?>

⚠️ Warning: PHP's default timezone is UTC. If your site serves users in a specific region, always call date_default_timezone_set() at the top of your script. Without it, timestamps on user comments might appear 8 hours off.

5. Real-World Date Recipes

PHP

<?php
// 1. Calculate age
function getAge(string $birthday): int {
    $born = new DateTime($birthday);
    $today = new DateTime();
    return $born->diff($today)->y;
}
echo getAge("2000-01-15");  // 26

// 2. Display relative time ("posted 3 days ago")
function timeAgo(string $datetime): string {
    $past = new DateTime($datetime);
    $now = new DateTime();
    $diff = $past->diff($now);

    if ($diff->y > 0)    return $diff->y . " year(s) ago";
    if ($diff->m > 0)    return $diff->m . " month(s) ago";
    if ($diff->d > 0)    return $diff->d . " day(s) ago";
    if ($diff->h > 0)    return $diff->h . " hour(s) ago";
    if ($diff->i > 0)    return $diff->i . " minute(s) ago";
    return "just now";
}

// 3. Generate an order number
function generateOrderNo(): string {
    return date("YmdHis") . mt_rand(1000, 9999);
}
echo generateOrderNo();  // 202606291430451234
?>

❓ よくある質問

Q date() or DateTime — which should I use?

A Use date() for simple one-line formatting. Reach for DateTime when you need to calculate date differences, iterate over date ranges, modify dates, or compare them.

Q Why is my time off by 8 hours?

A Timezone. Call date_default_timezone_set("Asia/Shanghai") in your script (or set date.timezone = Asia/Shanghai in your php.ini).

Q What does strtotime("2026/02/29") return?

A February 29 doesn't exist in 2026. strtotime auto-corrects and returns 2026-03-01. DateTime handles this the same way.

📖 まとめ

abs, ceil, floor, round, max, min, pow, sqrt are the most-used math functions
mt_rand() for fast random numbers; random_int() for cryptographically secure values
date("Y-m-d H:i:s") formats date-time via format characters
strtotime() converts human-readable phrases to timestamps
The DateTime class handles complex date operations (diff, modify, comparison)
Always set your timezone: date_default_timezone_set("Asia/Shanghai")

📝 練習問題

Use date() to output the current time in three different formats: ISO date (YYYY-MM-DD), a full human-readable format with weekday, and a compact 24-hour time.
Write a function getDaysBetween($date1, $date2) that uses DateTime to calculate the number of days between two dates.
Write a function isWeekend($date) that returns true if the given date falls on a Saturday or Sunday.

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