PHP Timezone Handling — Double-Booked Slots from Blind Spot

<?php

// time() returns the current Unix timestamp — an integer counting
// seconds elapsed since 00:00:00 UTC on 1 January 1970.
$currentTimestamp = time();
echo "Current Unix timestamp: " . $currentTimestamp . "\n";

// Because it's just a number, maths works perfectly on it.
$secondsInOneDay = 86400; // 60 seconds × 60 minutes × 24 hours

$tomorrowTimestamp  = $currentTimestamp + $secondsInOneDay;
$yesterdayTimestamp = $currentTimestamp - $secondsInOneDay;

echo "Tomorrow's timestamp:   " . $tomorrowTimestamp . "\n";
echo "Yesterday's timestamp:  " . $yesterdayTimestamp . "\n";

// The difference between two timestamps gives you elapsed seconds.
$orderPlacedAt  = 1711900000; // a past order's timestamp
$orderShippedAt = 1711986400; // when it was shipped

$secondsBetween = $orderShippedAt - $orderPlacedAt;
$hoursBetween   = $secondsBetween / 3600; // 3600 seconds in an hour

echo "Hours between order and shipment: " . $hoursBetween . "\n";

<?php

// date(format, timestamp) — timestamp is optional, defaults to now.
// Let's build up from simple to complex.

$rightNow = time(); // grab current timestamp once, use it everywhere

// --- Basic date components ---
$fourDigitYear   = date('Y', $rightNow); // e.g. 2025
$twoDigitYear    = date('y', $rightNow); // e.g. 25
$monthNumber     = date('m', $rightNow); // e.g. 04  (zero-padded)
$monthName       = date('F', $rightNow); // e.g. April
$shortMonthName  = date('M', $rightNow); // e.g. Apr
$dayOfMonth      = date('d', $rightNow); // e.g. 02  (zero-padded)
$dayOfWeekFull   = date('l', $rightNow); // e.g. Wednesday  (lowercase L)
$dayOfWeekShort  = date('D', $rightNow); // e.g. Wed

// --- Time components ---
$hours24         = date('H', $rightNow); // 00–23
$hours12         = date('h', $rightNow); // 01–12
$minutes         = date('i', $rightNow); // 00–59
$seconds         = date('s', $rightNow); // 00–59
$amOrPm          = date('A', $rightNow); // AM or PM

// --- Useful combinations ---
// ISO 8601 format — common in APIs and databases
$isoFormat = date('Y-m-d', $rightNow);
echo "ISO date:          " . $isoFormat . "\n";

// Human-friendly format
$friendlyFormat = date('l, F jS Y', $rightNow);
// 'j' = day without leading zero, 'S' = ordinal suffix (st, nd, rd, th)
echo "Friendly date:     " . $friendlyFormat . "\n";

// Time in 12-hour format
$timeFormat = date('h:i A', $rightNow);
echo "Current time:      " . $timeFormat . "\n";

// Full datetime — perfect for logging
$fullDatetime = date('Y-m-d H:i:s', $rightNow);
echo "Full datetime:     " . $fullDatetime . "\n";

// Formatting a specific past timestamp (a product launch date)
$productLaunchTimestamp = mktime(9, 0, 0, 3, 15, 2024); // 9:00 AM, March 15 2024
$launchDisplay = date('D, d M Y \a\t H:i', $productLaunchTimestamp);
// Backslashes escape literal letters so PHP doesn't treat them as format codes
echo "Product launch:    " . $launchDisplay . "\n";

<?php

// ── mktime(hour, minute, second, month, day, year) ──────────────────
// Build a timestamp for a specific moment you know in full.

$userBirthdayTimestamp = mktime(
    0,    // hour   (midnight)
    0,    // minute
    0,    // second
    8,    // month  (August)
    22,   // day
    1995  // year
);

echo "Birthday: " . date('F jS, Y', $userBirthdayTimestamp) . "\n";

// Calculate how many years ago that was
$yearsAgo = date('Y') - date('Y', $userBirthdayTimestamp);
echo "That was " . $yearsAgo . " years ago.\n\n";

// ── strtotime() — plain English date parsing ─────────────────────────
// Converts a natural-language date string into a Unix timestamp.

$nextMondayTimestamp    = strtotime('next Monday');
$twoWeeksFromNow        = strtotime('+2 weeks');
$startOfLastMonth       = strtotime('first day of last month');
$specificDateTimestamp  = strtotime('25 December 2025 08:00:00');

echo "Next Monday:           " . date('D, d M Y', $nextMondayTimestamp) . "\n";
echo "Two weeks from now:    " . date('D, d M Y', $twoWeeksFromNow) . "\n";
echo "First of last month:   " . date('D, d M Y', $startOfLastMonth) . "\n";
echo "Christmas 2025 8am:    " . date('D, d M Y H:i', $specificDateTimestamp) . "\n\n";

// ── Real-world example: subscription expiry ──────────────────────────
// A user signs up today, gets a 30-day free trial.

$signUpTimestamp    = time();
$trialEndTimestamp  = strtotime('+30 days', $signUpTimestamp);
// strtotime() accepts an optional base timestamp as a second argument.

$signUpDate  = date('Y-m-d', $signUpTimestamp);
$trialEndDate = date('Y-m-d', $trialEndTimestamp);

echo "Account created:  " . $signUpDate . "\n";
echo "Trial expires:    " . $trialEndDate . "\n";

// Check if the trial is still active
if (time() < $trialEndTimestamp) {
    echo "Status: Trial is ACTIVE.\n";
} else {
    echo "Status: Trial has EXPIRED.\n";
}

<?php

// ── Step 1: Always declare your timezone ─────────────────────────────
// Do this at the top of every PHP file that works with dates.
date_default_timezone_set('America/New_York');

// ── Step 2: Create DateTime objects ──────────────────────────────────

// Current moment in the server's default timezone
$now = new DateTime();
echo "Current time (NY):     " . $now->format('Y-m-d H:i:s T') . "\n";

// A specific past date
$eventDate = new DateTime('2025-07-04 18:00:00');
echo "Event date:            " . $eventDate->format('l, F jS Y \a\t g:i A') . "\n";

// DateTime in a DIFFERENT timezone using DateTimeZone
$tokyoTimezone = new DateTimeZone('Asia/Tokyo');
$tokyoNow      = new DateTime('now', $tokyoTimezone);
echo "Same moment in Tokyo:  " . $tokyoNow->format('Y-m-d H:i:s T') . "\n\n";

// ── Step 3: Date arithmetic with DateInterval ─────────────────────────
// DateInterval uses the ISO 8601 duration format:
// P = Period, Y = years, M = months, D = days, T = time separator,
// H = hours, M = minutes, S = seconds.

$invoiceDueDate = new DateTime('2025-04-02'); // today
$invoiceDueDate->modify('+30 days');          // add 30 days in place
echo "Invoice due:           " . $invoiceDueDate->format('Y-m-d') . "\n";

// Or use add() with a DateInterval object for more control
$subscriptionStart = new DateTime('2025-01-15');
$oneYear           = new DateInterval('P1Y');  // Period of 1 Year
$subscriptionEnd   = clone $subscriptionStart; // clone so we don't mutate the original
$subscriptionEnd->add($oneYear);

echo "Subscription start:    " . $subscriptionStart->format('Y-m-d') . "\n";
echo "Subscription end:      " . $subscriptionEnd->format('Y-m-d') . "\n\n";

// ── Step 4: Difference between two dates ─────────────────────────────
$projectStart    = new DateTime('2025-01-01');
$projectDeadline = new DateTime('2025-04-02');

// diff() returns a DateInterval showing the gap between two DateTimes
$durationInterval = $projectStart->diff($projectDeadline);

echo "Project duration: ";
echo $durationInterval->m . " months and " . $durationInterval->d . " days\n";
echo "Total days:       " . $durationInterval->days . " days\n";

// ── Step 5: Comparing two DateTime objects ────────────────────────────
$deadline = new DateTime('2025-03-31');
$today    = new DateTime('2025-04-02');

if ($today > $deadline) {
    echo "\nDeadline has passed.\n";
} else {
    echo "\nDeadline is still ahead.\n";
}

<?php

// ── Method 1: Validate strtotime() return value ──────────────────────
$input = '32-15-2025'; // completely invalid
$ts = strtotime($input);
if ($ts === false) {
    echo "Invalid date string: cannot parse.";
    // Handle error: log, return user feedback, etc.
} else {
    echo date('Y-m-d', $ts);
}

// ── Method 2: Use createFromFormat for strict parsing ─────────────────
$userInput = '2025-02-30'; // February 30th doesn't exist
$date = DateTime::createFromFormat('Y-m-d', $userInput);
// Check if the date exists AND the input exactly matched the format
if (!$date || $date->format('Y-m-d') !== $userInput) {
    echo "Invalid date: does not exist.\n";
} else {
    echo "Valid date: " . $date->format('Y-m-d');
}

// ── Method 3: checkdate() for component parts ─────────────────────────
echo checkdate(2, 29, 2024) ? 'Valid' : 'Invalid'; // 2024 is leap year → Valid
echo checkdate(2, 29, 2025) ? 'Valid' : 'Invalid'; // 2025 is not → Invalid

// ── Real-world: parsing a CSV with mixed date formats ─────────────────
$csvDate = '04/02/2025'; // ambiguous: US vs EU?
$formats = ['m/d/Y', 'd/m/Y', 'Y-m-d'];
$parsed = null;
foreach ($formats as $fmt) {
    $d = DateTime::createFromFormat($fmt, $csvDate);
    if ($d && $d->format($fmt) === $csvDate) {
        $parsed = $d;
        break;
    }
}
if ($parsed) {
    echo "Parsed as: " . $parsed->format('Y-m-d');
} else {
    echo "Unable to parse date.";
}