Miscalculation of a date using microseconds

[axi]

Description

The following code:

<?php
date_default_timezone_set("UTC");

$beginningOfYear = new \DateTimeImmutable('1st january this year')
    ->setTime(0, 0);
$endOfYear = new \DateTimeImmutable('31th december this year')
    ->setTime(23, 59, 59, 999999);


$uuThisYear = (int) $endOfYear->format('Uu') - (int) $beginningOfYear->format('Uu');
$sThisYear = (int) $endOfYear->format('U') - (int) $beginningOfYear->format('U');
$usThisYear = (int) $endOfYear->format('u') - (int) $beginningOfYear->format('u');

$uuThisYear2 = (int) ($sThisYear . $usThisYear);

echo "<pre>" . var_export([
    'from' => $beginningOfYear,
    'fromMs' => (int) $beginningOfYear->format('Uu'),
    'to' => $endOfYear,
    'toMs' => (int) $endOfYear->format('Uu'),
    'nbMicrosecondsBetweenToAndFrom' => $uuThisYear,
    'nbMicrosecondsBetweenToAndFromOtherCalculation' => $uuThisYear2,
    'equal' => $uuThisYear === $uuThisYear2,
    'nbSecondsBetweenToAndFrom' => $sThisYear,
    'nbUsecondsBetweenToAndFrom' => $usThisYear,
    'shouldEqualToFromButDoesntWork' => (clone $endOfYear)->modify('-' . $uuThisYear . ' µsec'),
    'shouldEqualToFromWorks' => (clone $endOfYear)
        ->modify('-' . $sThisYear . ' seconds')
        ->modify('-' . $usThisYear . ' microseconds'),
], true);

Resulted in this output:

array (
  'from' => 
  \DateTimeImmutable::__set_state(array(
     'date' => '2025-01-01 00:00:00.000000',
     'timezone_type' => 3,
     'timezone' => 'UTC',
  )),
  'fromMs' => 1735689600000000,
  'to' => 
  \DateTimeImmutable::__set_state(array(
     'date' => '2025-12-31 23:59:59.999999',
     'timezone_type' => 3,
     'timezone' => 'UTC',
  )),
  'toMs' => 1767225599999999,
  'nbMicrosecondsBetweenToAndFrom' => 31535999999999,
  'nbMicrosecondsBetweenToAndFromOtherCalculation' => 31535999999999,
  'equal' => true,
  'nbSecondsBetweenToAndFrom' => 31535999,
  'nbUsecondsBetweenToAndFrom' => 999999,
  'shouldEqualToFromButDoesntWork' => 
  \DateTimeImmutable::__set_state(array(
     'date' => '2026-01-01 09:59:59.999998',
     'timezone_type' => 3,
     'timezone' => 'UTC',
  )),
  'shouldEqualToFromWorks' => 
  \DateTimeImmutable::__set_state(array(
     'date' => '2025-01-01 00:00:00.000000',
     'timezone_type' => 3,
     'timezone' => 'UTC',
  )),
)

But I expected this output instead:

array (
  'from' => 
  \DateTimeImmutable::__set_state(array(
     'date' => '2025-01-01 00:00:00.000000',
     'timezone_type' => 3,
     'timezone' => 'UTC',
  )),
  'fromMs' => 1735689600000000,
  'to' => 
  \DateTimeImmutable::__set_state(array(
     'date' => '2025-12-31 23:59:59.999999',
     'timezone_type' => 3,
     'timezone' => 'UTC',
  )),
  'toMs' => 1767225599999999,
  'nbMicrosecondsBetweenToAndFrom' => 31535999999999,
  'nbMicrosecondsBetweenToAndFromOtherCalculation' => 31535999999999,
  'equal' => true,
  'nbSecondsBetweenToAndFrom' => 31535999,
  'nbUsecondsBetweenToAndFrom' => 999999,
  'shouldEqualToFromButDoesntWork' => 
  \DateTimeImmutable::__set_state(array(
     'date' => '2025-01-01 00:00:00.000000',
     'timezone_type' => 3,
     'timezone' => 'UTC',
  )),
  'shouldEqualToFromWorks' => 
  \DateTimeImmutable::__set_state(array(
     'date' => '2025-01-01 00:00:00.000000',
     'timezone_type' => 3,
     'timezone' => 'UTC',
  )),
)

Check the "shouldEqualToFromButDoesntWork" value.

PHP Version

8.4.14

Operating System

Mint 22.2 Zara

[beryllium]

From helping axi troubleshoot, here is my attempt at an example case:

<?php

function out(string $prefix, DateTimeImmutable $date) {
    echo $prefix . ': ' . $date->format('c') . '(' . $date->format('Uu') . ')' . PHP_EOL;
}

$today = new \DateTimeImmutable();
out('Today', $today);

$yesterday = $today->modify('1 day ago');
out('Yesterday', $yesterday);

$oneYearAgo = $today->modify('1 year ago');
out('One year ago', $oneYearAgo);

$microseconds = 1_000_000 * 60 * 60 * 24 * 7; // seven days ago in microseconds
$microsecondsAgo = $today->modify('-' . $microseconds . ' microseconds');
out(number_format($microseconds) . ' microseconds ago', $microsecondsAgo);

echo "*************************" . PHP_EOL;
echo "** This is the outlier **" . PHP_EOL . PHP_EOL;
$microseconds = 1_000_000 * 60 * 60 * 24 * 365; // one year ago in microseconds
$microsecondsAgo = $today->modify('-' . $microseconds . ' microseconds');
out(number_format($microseconds) . ' microseconds ago', $microsecondsAgo);
echo PHP_EOL . "*************************" . PHP_EOL;

$seconds = 1 * 60 * 60 * 24 * 365; // one year ago in seconds
$secondsAgo = $today->modify('-' . $seconds . ' seconds');
out(number_format($seconds) . ' seconds ago', $secondsAgo);

// An approach that works, and may be the "correct" way to do it,
// but feels like it shouldn't be necessary:

$msDelta = 31_536_000_000_000; // one year in microseconds
$sDelta = intdiv($msDelta, 1_000_000); // one year in seconds
$msRemainder = $msDelta % 1_000_000; // remainder in microseconds

out(
    '1 year ago, first subtracting seconds, then subtracting remainder of microseconds',
    $today
      ->modify('-' . $sDelta . ' seconds')
      ->modify('-' . $msRemainder . ' microseconds')
);