Calendar for multiple post types; needs cleanup (circa 2010).

gistfile1.php

<?php

function ucc_get_calendar( $post_types = '', $initial = true, $echo = true ) {
	global $wpdb, $m, $monthnum, $year, $wp_locale, $posts;

	// Set and verify post types to fetch
	if ( empty( $post_types ) || !is_array( $post_types ) ) {
		$args     = array(
			'public' => true,
			'_builtin' => false,
		);
		$output   = 'names';
		$operator = 'and';

		$post_types = get_post_types( $args, $output, $operator );
		$post_types = array_merge( $post_types, array( 'post' ) );
	} else {
		// Trust but verify
		$my_post_types = array();
		foreach ( $post_types as $post_type ) {
			if ( post_type_exists( $post_type ) )
				$my_post_types[] = $post_type;
		}
		$post_types = $my_post_types;
	}

	// Set up for cache request
	$post_types_key = implode( '', $post_types );
	$cache          = array();
	$key            = md5( $m . $monthnum . $year . $post_types_key );

	// Check cache for existing calendar
	if ( $cache = wp_cache_get( 'get_calendar', 'calendar' ) ) {
		if ( is_array( $cache ) && isset( $cache[$key] ) ) {
			remove_filter( 'get_calendar', 'ucc_get_calendar_filter' );
			$output = apply_filters( 'get_calendar', $cache[$key] );
			add_filter( 'get_calendar', 'ucc_get_calendar_filter' );
			if ( $echo ) {
				echo $output;
				return;
			} else {
				return $output;
			}
		}
	}

	if ( !is_array( $cache ) )
		$cache = array();

	// Set up for SQL IN()
	$post_types_in  = array();
	foreach ( $post_types as $post_type ) {
		$post_types_in[] = "'" . esc_sql( $post_type ) . "'";
	}
	$post_types_in  = implode( ', ', $post_types_in );

	// Quick check. If we have no posts at all, abort!
	if ( !$posts ) {
		$sql = "SELECT 1 as test FROM $wpdb->posts WHERE post_type IN ( $post_types_in ) AND post_status = 'publish' LIMIT 1";
		$gotsome = $wpdb->get_var( $sql );
		if ( !$gotsome ) {
			$cache[$key] = '';
			wp_cache_set( 'get_calendar', $cache, 'calendar' );
			return;
		}
	}

	if ( isset( $_GET['w'] ) )
		$w = '' . intval( $_GET['w'] );

	// week_begins = 0 stands for Sunday
	$week_begins = intval( get_option( 'start_of_week' ) );

	// Let's figure out when we are
	if ( !empty( $monthnum ) && !empty( $year ) ) {
		$thismonth = '' . zeroise( intval( $monthnum ), 2 );
		$thisyear  = ''.intval( $year );
	} elseif ( !empty( $w ) ) {
		// We need to get the month from MySQL
		$thisyear  = '' . intval( substr( $m, 0, 4 ) );
		$d         = ( ( $w - 1 ) * 7 ) + 6; //it seems MySQL's weeks disagree with PHP's
		$thismonth = $wpdb->get_var( "SELECT DATE_FORMAT( ( DATE_ADD( '${thisyear}0101', INTERVAL $d DAY ) ), '%m' ) " );
	} elseif ( !empty( $m ) ) {
		$thisyear  = '' . intval( substr( $m, 0, 4 ) );
		if ( strlen( $m ) < 6 )
			$thismonth = '01';
		else
			$thismonth = '' . zeroise( intval( substr( $m, 4, 2 ) ), 2 );
	} else {
		$thisyear  = gmdate( 'Y', current_time( 'timestamp' ) );
		$thismonth = gmdate( 'm', current_time( 'timestamp' ) );
	}

	$unixmonth = mktime( 0, 0, 0, $thismonth, 1, $thisyear );

	// Get the next and previous month and year with at least one post
	$previous = $wpdb->get_row( "SELECT DISTINCT MONTH( post_date ) AS month, YEAR( post_date ) AS year
		FROM $wpdb->posts
		WHERE post_date < '$thisyear-$thismonth-01'
		AND post_type IN ( $post_types_in ) AND post_status = 'publish'
		ORDER BY post_date DESC
		LIMIT 1" );
	$next = $wpdb->get_row( "SELECT DISTINCT MONTH( post_date ) AS month, YEAR( post_date ) AS year
		FROM $wpdb->posts
		WHERE post_date > '$thisyear-$thismonth-01'
		AND MONTH( post_date ) != MONTH( '$thisyear-$thismonth-01' )
		AND post_type IN ( $post_types_in ) AND post_status = 'publish'
		ORDER BY post_date ASC
		LIMIT 1" );

	/* translators: Calendar caption: 1: month name, 2: 4-digit year */
	$calendar_caption = _x( '%1$s %2$s', 'calendar caption' );
	$calendar_output = '<table id="wp-calendar" summary="' . esc_attr__( 'Calendar' ) . '">
	<caption>' . sprintf( $calendar_caption, $wp_locale->get_month( $thismonth ), date( 'Y', $unixmonth ) ) . '</caption>
	<thead>
	<tr>';

	$myweek = array();

	for ( $wdcount = 0 ; $wdcount <= 6 ; $wdcount++ ) {
		$myweek[] = $wp_locale->get_weekday( ( $wdcount + $week_begins ) % 7 );
	}

	foreach ( $myweek as $wd ) {
		$day_name = ( true == $initial ) ? $wp_locale->get_weekday_initial( $wd ) : $wp_locale->get_weekday_abbrev( $wd );
		$wd = esc_attr( $wd );
		$calendar_output .= "\n\t\t<th scope=\"col\" title=\"$wd\">$day_name</th>";
	}

	$calendar_output .= '
	</tr>
	</thead>

	<tfoot>
	<tr>';

	if ( $previous ) {
		$calendar_output .= "\n\t\t" . '<td colspan="3" id="prev"><a href="' . get_month_link( $previous->year, $previous->month ) . '" title="' . sprintf( __( 'View posts for %1$s %2$s' ), $wp_locale->get_month( $previous->month ), date( 'Y', mktime( 0, 0, 0, $previous->month, 1, $previous->year ) ) ) . '">&laquo; ' . $wp_locale->get_month_abbrev( $wp_locale->get_month( $previous->month ) ) . '</a></td>';
	} else {
		$calendar_output .= "\n\t\t" . '<td colspan="3" id="prev" class="pad">&nbsp;</td>';
	}

	$calendar_output .= "\n\t\t" . '<td class="pad">&nbsp;</td>';

	if ( $next ) {
		$calendar_output .= "\n\t\t" . '<td colspan="3" id="next"><a href="' . get_month_link( $next->year, $next->month ) . '" title="' . esc_attr( sprintf( __( 'View posts for %1$s %2$s' ), $wp_locale->get_month( $next->month ), date( 'Y', mktime( 0, 0, 0, $next->month, 1, $next->year ) ) ) ) . '">' . $wp_locale->get_month_abbrev( $wp_locale->get_month( $next->month ) ) . ' &raquo;</a></td>';
	} else {
		$calendar_output .= "\n\t\t" . '<td colspan="3" id="next" class="pad">&nbsp;</td>';
	}

	$calendar_output .= '
	</tr>
	</tfoot>

	<tbody>
	<tr>';

	// Get days with posts
	$dayswithposts = $wpdb->get_results( "SELECT DISTINCT DAYOFMONTH( post_date )
		FROM $wpdb->posts WHERE MONTH( post_date ) = '$thismonth'
		AND YEAR( post_date ) = '$thisyear'
		AND post_type IN ( $post_types_in ) AND post_status = 'publish'
		AND post_date < '" . current_time( 'mysql' ) . '\'', ARRAY_N );
	if ( $dayswithposts ) {
		foreach ( (array) $dayswithposts as $daywith ) {
			$daywithpost[] = $daywith[0];
		}
	} else {
		$daywithpost = array();
	}

	if ( strpos( $_SERVER['HTTP_USER_AGENT'], 'MSIE' ) !== false || stripos( $_SERVER['HTTP_USER_AGENT'], 'camino' ) !== false || stripos( $_SERVER['HTTP_USER_AGENT'], 'safari' ) !== false )
		$ak_title_separator = "\n";
	else
		$ak_title_separator = ', ';

	$ak_titles_for_day = array();
	$ak_post_titles = $wpdb->get_results( "SELECT ID, post_title, DAYOFMONTH( post_date ) as dom "
		. "FROM $wpdb->posts "
		. "WHERE YEAR( post_date ) = '$thisyear' "
		. "AND MONTH( post_date ) = '$thismonth' "
		. "AND post_date < '" . current_time( 'mysql' ) . "' "
		. "AND post_type IN ( $post_types_in ) AND post_status = 'publish'"
	);
	if ( $ak_post_titles ) {
		foreach ( (array) $ak_post_titles as $ak_post_title ) {

				$post_title = esc_attr( apply_filters( 'the_title', $ak_post_title->post_title, $ak_post_title->ID ) );

				if ( empty( $ak_titles_for_day['day_' . $ak_post_title->dom] ) )
					$ak_titles_for_day['day_'.$ak_post_title->dom] = '';
				if ( empty( $ak_titles_for_day["$ak_post_title->dom"] ) ) // first one
					$ak_titles_for_day["$ak_post_title->dom"] = $post_title;
				else
					$ak_titles_for_day["$ak_post_title->dom"] .= $ak_title_separator . $post_title;
		}
	}

	// See how much we should pad in the beginning
	$pad = calendar_week_mod( date( 'w', $unixmonth ) - $week_begins );
	if ( 0 != $pad )
		$calendar_output .= "\n\t\t" . '<td colspan="' . esc_attr( $pad ) . '" class="pad">&nbsp;</td>';

	$daysinmonth = intval( date( 't', $unixmonth ) );
	for ( $day = 1 ; $day <= $daysinmonth ; ++$day ) {
		if ( isset( $newrow ) && $newrow )
			$calendar_output .= "\n\t</tr>\n\t<tr>\n\t\t";
		$newrow = false;

		if ( $day == gmdate( 'j', current_time( 'timestamp' ) ) && $thismonth == gmdate( 'm', current_time( 'timestamp' ) ) && $thisyear == gmdate( 'Y', current_time( 'timestamp' ) ) )
			$calendar_output .= '<td id="today">';
		else
			$calendar_output .= '<td>';

		if ( in_array( $day, $daywithpost ) ) // any posts today?
				$calendar_output .= '<a href="' . get_day_link( $thisyear, $thismonth, $day ) . "\" title=\"" . esc_attr( $ak_titles_for_day[$day] ) . "\">$day</a>";
		else
			$calendar_output .= $day;
		$calendar_output .= '</td>';

		if ( 6 == calendar_week_mod( date( 'w', mktime( 0, 0, 0, $thismonth, $day, $thisyear ) ) - $week_begins ) )
			$newrow = true;
	}

	$pad = 7 - calendar_week_mod( date( 'w', mktime( 0, 0, 0, $thismonth, $day, $thisyear ) ) - $week_begins );
	if ( $pad != 0 && $pad != 7 )
		$calendar_output .= "\n\t\t" . '<td class="pad" colspan="' . esc_attr( $pad ) . '">&nbsp;</td>';

	$calendar_output .= "\n\t</tr>\n\t</tbody>\n\t</table>";

	$cache[$key] = $calendar_output;
	wp_cache_set( 'get_calendar', $cache, 'calendar' );

	remove_filter( 'get_calendar', 'ucc_get_calendar_filter' );
	$output = apply_filters( 'get_calendar', $calendar_output );
	add_filter( 'get_calendar', 'ucc_get_calendar_filter' );

	if ( $echo )
		echo $output;
	else
		return $output;
}

function ucc_get_calendar_filter( $content ) {
	$output = ucc_get_calendar( '', '', false );
	return $output;
}
add_filter( 'get_calendar', 'ucc_get_calendar_filter', 10, 2 );