diff --git a/Sources/Time.php b/Sources/Time.php
index 0b028ec262..f094333bcf 100644
--- a/Sources/Time.php
+++ b/Sources/Time.php
@@ -439,6 +439,18 @@ public function __isset(string $prop): bool
 		}
 	}
 
+	/**
+	 * Like \DateTime::diff(), but returns a TimeInterval object.
+	 *
+	 * @param \DateTimeInterface $target The date to compare to.
+	 * @param bool $absolute Whether to force the interval to be positive.
+	 * @return TimeInterval The difference between the two dates.
+	 */
+	public function diff(\DateTimeInterface $target, bool $absolute = false): TimeInterval
+	{
+		return TimeInterval::createFromDateInterval(parent::diff($target, $absolute));
+	}
+
 	/**
 	 * Like DateTime::format(), except that it can accept either DateTime format
 	 * specifiers or strftime format specifiers (but not both at once).
diff --git a/Sources/TimeInterval.php b/Sources/TimeInterval.php
index 0131cc2f2d..bfd4856300 100644
--- a/Sources/TimeInterval.php
+++ b/Sources/TimeInterval.php
@@ -20,6 +20,88 @@
  */
 class TimeInterval extends \DateInterval implements \Stringable
 {
+	/*******************
+	 * Public properties
+	 *******************/
+
+	/**
+	 * @var int
+	 *
+	 * Number of years.
+	 */
+	public int $y;
+
+	/**
+	 * @var int
+	 *
+	 * Number of months.
+	 */
+	public int $m;
+
+	/**
+	 * @var int
+	 *
+	 * Number of days.
+	 */
+	public int $d;
+
+	/**
+	 * @var int
+	 *
+	 * Number of hours.
+	 */
+	public int $h;
+
+	/**
+	 * @var int
+	 *
+	 * Number of minutes.
+	 */
+	public int $i;
+
+	/**
+	 * @var int
+	 *
+	 * Number of seconds.
+	 */
+	public int $s;
+
+	/**
+	 * @var float
+	 *
+	 * Number of microseconds, as a fraction of a second.
+	 */
+	public float $f;
+
+	/**
+	 * @var int
+	 *
+	 * 1 if the interval represents a negative time period and 0 otherwise.
+	 */
+	public int $invert;
+
+	/**
+	 * @var mixed
+	 *
+	 * If the object was created by Time::diff(), then this is the total number
+	 * of full days between the start and end dates. Otherwise, false.
+	 */
+	public mixed $days;
+
+	/**
+	 * @var bool
+	 *
+	 * Whether the object was created by TimeInterval::createFromDateString().
+	 */
+	public bool $from_string;
+
+	/**
+	 * @var string
+	 *
+	 * The string used as argument to TimeInterval::createFromDateString().
+	 */
+	public string $date_string;
+
 	/****************
 	 * Public methods
 	 ****************/
@@ -128,36 +210,28 @@ public function __construct(string $duration)
 			$can_be_fractional = false;
 		}
 
-		if (!isset($frac['prop'])) {
-			// If we have no fractional values, construction is easy.
-			parent::__construct($duration);
-		} else {
-			// Rebuild $duration without the fractional value.
-			$duration = 'P';
-
-			foreach (array_reverse($props) as $prop => $info) {
-				if ($prop === 'h') {
-					$duration .= 'T';
-				}
-
-				if (!empty($matches[$prop])) {
-					$duration .= $matches[$prop] . $info['unit'];
-				}
-			}
-
-			// Construct.
-			parent::__construct(rtrim($duration, 'PT'));
-
-			// Finally, set the fractional value.
+		// Add the fractional value where appropriate.
+		if (isset($frac['prop'])) {
 			$this->{$frac['prop']} += $frac['value'];
 		}
+
+		// Set our properties.
+		$this->y = $matches['y'] ?? 0;
+		$this->m = $matches['m'] ?? 0;
+		$this->d = $matches['d'] ?? 0;
+		$this->h = $matches['h'] ?? 0;
+		$this->i = $matches['i'] ?? 0;
+		$this->s = $matches['s'] ?? 0;
+		$this->f = $matches['f'] ?? 0.0;
+		$this->days = false;
+		$this->from_string = false;
 	}
 
 	/**
-	 * Formats the object as a string so it can be reconstructed later.
+	 * Formats the interval as a string so it can be reconstructed later.
 	 *
 	 * @return string A ISO 8601 duration string suitable for reconstructing
-	 *    this object.
+	 *    this interval.
 	 */
 	public function __toString(): string
 	{
@@ -187,10 +261,11 @@ public function __toString(): string
 	}
 
 	/**
-	 * Formats the object as a string that can be parsed by strtotime().
+	 * Formats this interval as a string that can be parsed by
+	 * TimeInterval::createFromDateString().
 	 *
-	 * @return string A strtotime parsable string suitable for reconstructing
-	 *    this object.
+	 * @return string A parsable string suitable for reconstructing
+	 *    this interval.
 	 */
 	public function toParsable(): string
 	{
@@ -230,6 +305,71 @@ public function toParsable(): string
 		return implode(' ', $result);
 	}
 
+	/**
+	 * Formats the interval as a human-readable string in the current user's
+	 * language.
+	 *
+	 * @param array $format_chars Properties to include in the output.
+	 *    Allowed values in this array: 'y', 'm', 'd', 'h', 'i', 's', 'f', 'a'.
+	 *    Note that when 'f' is included, it will always be combined with 's' in
+	 *    order to produce a single float value in the output.
+	 * @return string A human-readable string.
+	 */
+	public function localize(array $format_chars = ['y', 'm', 'd']): string
+	{
+		$result = [];
+
+		$txt_keys = [
+			'y' => 'number_of_years',
+			'm' => 'number_of_months',
+			'd' => 'number_of_days',
+			'a' => 'number_of_days',
+			'h' => 'number_of_hours',
+			'i' => 'number_of_minutes',
+			's' => 'number_of_seconds',
+			'f' => 'number_of_seconds',
+		];
+
+		foreach ($format_chars as $c) {
+			// Don't include a bunch of useless "0 <unit>" substrings.
+			if (empty($this->{$c}) || !isset($txt_keys[$c])) {
+				continue;
+			}
+
+			switch ($c) {
+				case 'f':
+					if (!in_array('s', $format_chars)) {
+						$result[] = Lang::getTxt($txt_keys[$c], [(float) $this->s + (float) $this->f]);
+					}
+					break;
+
+				case 's':
+					if (in_array('f', $format_chars)) {
+						$result[] = Lang::getTxt($txt_keys[$c], [(float) $this->s + (float) $this->f]);
+					} else {
+						$result[] = Lang::getTxt($txt_keys[$c], [$this->s]);
+					}
+					break;
+
+				default:
+					$result[] = Lang::getTxt($txt_keys[$c], [$this->{$c}]);
+					break;
+			}
+		}
+
+		// If all requested properties were empty, output a single "0 <unit>"
+		// for the smallest unit requested.
+		if (empty($result)) {
+			foreach ($txt_keys as $c => $k) {
+				if (in_array($c, $format_chars)) {
+					$result = [Lang::getTxt($txt_keys[$c], [0])];
+				}
+			}
+		}
+
+		return Lang::sentenceList($result);
+	}
+
 	/**
 	 * Converts this interval to a number of seconds.
 	 *
@@ -252,6 +392,37 @@ public function toSeconds(\DateTimeInterface $when): int|float
 		return ($later->format($fmt) - $when->format($fmt));
 	}
 
+	/**
+	 * Formats the interval using an arbitrary format string.
+	 *
+	 * @param string $format The format string. Accepts all the same format
+	 *    specifiers that \DateInterval::format() accepts.
+	 * @return string The formatted interval.
+	 */
+	public function format(string $format): string
+	{
+		return strtr($format, [
+			'%Y' => sprintf('%02d', $this->y),
+			'%y' => sprintf('%01d', $this->y),
+			'%M' => sprintf('%02d', $this->m),
+			'%m' => sprintf('%01d', $this->m),
+			'%D' => sprintf('%02d', $this->d),
+			'%d' => sprintf('%01d', $this->d),
+			'%a' => is_int($this->days) ? sprintf('%01d', $this->days) : '(unknown)',
+			'%H' => sprintf('%02d', $this->h),
+			'%h' => sprintf('%01d', $this->h),
+			'%I' => sprintf('%02d', $this->i),
+			'%i' => sprintf('%01d', $this->i),
+			'%S' => sprintf('%02d', $this->s),
+			'%s' => sprintf('%01d', $this->s),
+			'%F' => substr(sprintf('%06f', $this->f), 2),
+			'%f' => ltrim(sprintf('%06f', $this->f), '0.'),
+			'%R' => $this->invert ? '-' : '+',
+			'%r' => $this->invert ? '-' : '',
+			'%%' => '%',
+		]);
+	}
+
 	/***********************
 	 * Public static methods
 	 ***********************/
@@ -266,12 +437,36 @@ public static function createFromDateInterval(\DateInterval $object): static
 	{
 		$new = new TimeInterval('P0D');
 
-		foreach (['y', 'm', 'd', 'h', 'i', 's', 'f', 'invert'] as $prop) {
-			$new->{$prop} = $object->{$prop};
+		foreach (get_object_vars($object) as $prop => $value) {
+			$new->{$prop} = $value;
 		}
 
 		return $new;
 	}
+
+	/**
+	 * Creates a TimeInterval from the relative parts of a date string.
+	 *
+	 * @param string $datetime A date with relative parts.
+	 * @return TimeInterval A TimeInterval object.
+	 */
+	public static function createFromDateString(string $datetime): static
+	{
+		$object = parent::createFromDateString($datetime);
+
+		$new = self::createFromDateInterval($object);
+
+		$new->y = (int) $object->format('%y');
+		$new->m = (int) $object->format('%m');
+		$new->d = abs((int) $object->format('%d'));
+		$new->h = (int) $object->format('%h');
+		$new->i = (int) $object->format('%i');
+		$new->s = (int) $object->format('%s');
+		$new->f = (float) ('0.' . $object->format('%F'));
+		$new->invert = (int) ($object->format('%d') < 0);
+
+		return $new;
+	}
 }
 
 ?>
\ No newline at end of file