Some utility functions for working with dates in PHP

Date.php

<?php

class Date
{
	/**
	 * Returns time difference between two timestamps, in human readable format.
	 *
	 * @param int $time1 A timestamp.
	 * @param int $time2 A timestamp, defaults to the current time.
	 * @param string $output Formatting string specifying which parts of the date to return in the array.
	 * @return string|array
	 */
	public static function timespan($time1, $time2 = null, $output = 'years,months,weeks,days,hours,minutes,seconds')
	{
		// Array with the output formats
		$output = preg_split('/[^a-z]+/', strtolower((string) $output));

		// Invalid output
		if (empty($output)) {
			return false;
		}

		// Make the output values into keys
		extract(array_flip($output), EXTR_SKIP);

		// Default values
		$time1  = max(0, (int) $time1);
		$time2  = empty($time2) ? time() : max(0, (int) $time2);

		// Calculate timespan (seconds)
		$timespan = abs($time1 - $time2);

		// All values found using Google Calculator.
		// Years and months do not match the formula exactly, due to leap years.

		// Years ago, 60 * 60 * 24 * 365
		if (isset($years) ) {
			$timespan -= 31556926 * ($years = (int) floor($timespan / 31556926));
		}

		// Months ago, 60 * 60 * 24 * 30
		if (isset($months)) {
			$timespan -= 2629744 * ($months = (int) floor($timespan / 2629743.83));
		}

		// Weeks ago, 60 * 60 * 24 * 7
		if (isset($weeks)) {
			$timespan -= 604800 * ($weeks = (int) floor($timespan / 604800));
		}

		// Days ago, 60 * 60 * 24
		if (isset($days)) {
			$timespan -= 86400 * ($days = (int) floor($timespan / 86400));
		}

		// Hours ago, 60 * 60
		if (isset($hours)) {
			$timespan -= 3600 * ($hours = (int) floor($timespan / 3600));
		}

		// Minutes ago, 60
		if (isset($minutes)) {
			$timespan -= 60 * ($minutes = (int) floor($timespan / 60));
		}

		// Seconds ago, 1
		if (isset($seconds)) {
			$seconds = $timespan;
		}

		// Remove the variables that cannot be accessed
		unset($timespan, $time1, $time2);

		// Deny access to these variables
		$deny = array('deny', 'key', 'difference', 'output');

		// Return the difference
		$difference = array();
		foreach ($output as $key) {
			if (isset($$key) AND !in_array($key, $deny)) {
				// Add requested key to the output
				$difference[$key] = $$key;
			}
		}

		// Invalid output formats string
		if (empty($difference)) {
			return false;
		}

		// If only one output format was asked, don't put it in an array
		if (count($difference) === 1) {
			return current($difference);
		}

		// Return array
		return $difference;
	}

	/**
	 * Expands upon the functionality provided by Date::timespan(), such that when provided
	 * with two timestamps (or one and using time() as the second), it will intelligently
	 * predict the human readable date format to use, based on the length of the timespan.
	 * Consequently the time string returned is more of an approximation - it will in some
	 * cases be less than the length of the actual period, but never more.
	 *
	 * timespan < 1 minute: 'X seconds'
	 * timespan < 1 hour: 'X minutes'
	 * timespan < 1 day: 'X hours'
	 * timespan < 1 week: 'X days'
	 * timespan < 1 month: 'X weeks'
	 * timespan < 1 year: 'X months'
	 *
	 * It will also unpluralise if the unit is 1, so 1 seconds becomes 1 second.
	 *
	 * @param int $time1 A timestamp.
	 * @param int|false $time2 A timestamp, defaults to the current time (passing null). If set to false, the
	 * first parameter will be treated as the period on its own.
	 * @return string Human readable format
	 */
	public static function humanFormat($time1, $time2 = null)
	{
		// Default values
		$time1  = max(0, (int) $time1);

		if($time2 !== false) {
			$time2  = empty($time2) ? time() : max(0, (int) $time2);

			// Calculate timespan (seconds)
			$period = abs($time1 - $time2);

		} else {
			$period = $time1;
		}

		$format = 'seconds';
		if ($period > 31556926) {
			// More than one year
			$format = 'years';
		}
		elseif ($period > 2629744) {
			// More than one month
			$format = 'months';
		}
		elseif ($period > 604800) {
			// More than one week
			$format = 'weeks';
		}
		elseif ($period > 86400) {
			// More than one day
			$format = 'days';
		}
		elseif ($period > 3600) {
			// More than one hour
			$format = 'hours';
		}
		elseif ($period > 60) {
			// More than one minute
			$format = 'minutes';
		}

		if($time2 !== false) {
			// Get timespan output
			$timespan = self::timespan($time1, $time2, $format);
		} else {
			$timespan = $time1;
		}

		// Remove the s
		if($timespan == 1) {
			$format = substr($format, 0, -1);
		}

		return $timespan . ' ' . $format;
	}
}