Gets date ranges for the BETWEEN operator in a WordPress meta query

gistfile1.php

<?php
/**
 * Gets date ranges for the BETWEEN operator in a WordPress meta query.
 * WordPress minimum requirements don't let you
 * use php DateTime or (some) relative formats for strtotime() which were introduced in php 5.3.
 *
 * @param string|array $args Arguments.
 * @return array Array with two dates.
 */
function get_between_clause_dates( $args ) {

	$defaults = array(
		'type' => 'month', // month - year, month, day
		'start_from' => 'today', // today, type - Start from today, or from first/last day of month/year.
		'future_past' => 'past', // past, future - Past or future date range.
		'offset' => 0, // 0 = current month (1 = one month from current date etc...).
		'duration' => 1, // duration of 1 month (1 or higher).
	);

	$args = wp_parse_args( $args, $defaults );

	if ( !in_array( $args['type'], array( 'year', 'month', 'day' ) ) ) {
		$args['type'] = 'month';
	}

	if ( !in_array( $args['start_from'], array( 'today', 'type' ) ) ) {
		$args['start_from'] = 'today';
	}

	if ( !in_array( $args['future_past'], array( 'future', 'past' ) ) ) {
		$args['future_past'] = 'past';
	}

	$args['offset'] = absint( $args['offset'] );
	$args['duration'] = absint( $args['duration'] );
	$args['duration'] = ( $args['duration'] ) ? $args['duration'] : 1;

	extract( $args );

	// start from today
	$start_date = getdate();

	if ( ( 'type' == $start_from ) && ( 'day' != $type ) ) {

		// start from this year, month

		if ( 'past' == $future_past ) {

			// into the past (start from last day of current type)
			switch ( $type ) {

			case 'year':
				$start_date['mon'] = 12;
				$start_date['mday'] = 31;
				break;

			case 'month':
				// get last day of month with offset
				$last_day = date( "t",  mktime( 0, 0, 0, $start_date['mon'], 1,  $start_date['year'] ) );
				$start_date['mday'] = $last_day;
				break;

			}

		} else {

			// into the future (start from first day of current type)
			switch ( $type ) {
			case 'year': $start_date['mon'] = $start_date['mday'] = 1;
			case 'month': $start_date['mday'] = 1; break;
			}
		}

	}

	if ( $offset > 0 ) {
		$start_date = get_date_offset( $start_date, $args );
	}

	$args['offset'] = $duration;
	$end_date = get_date_offset( $start_date, $args, true );

	$start_date = $start_date['year'] . '-' . zeroise( $start_date['mon'], 2 ) . '-' . zeroise( $start_date['mday'], 2 );
	$end_date = $end_date['year'] . '-' . zeroise( $end_date['mon'], 2 ) . '-' . zeroise( $end_date['mday'], 2 );

	if ( 'past' == $future_past ) {
		return array( $end_date, $start_date );
	} else {
		return array( $start_date, $end_date );
	}
}

/**
 * Gets a date with an offset.
 *
 * @param array   $date     Date array from the getdate() function.
 * @param array   $args     Arguments.
 * @param bool    $end_date If this is an end date or a start date. Default: false.
 * @return array            getdate() array with an offset date.
 */
function get_date_offset( $date, $args, $end_date = false ) {

	extract( $args );

	if ( 'past' == $future_past ) {

		switch ( $type ) {

		case 'year':
		case 'month':
			$_type = ( 'month' == $type ) ? 'mon' : $type;
			$date[ $_type ] = $date[ $_type ] - $offset;
			if ( ( $start_from == 'type' ) && $end_date ) { // end date in the past
				$date['mday'] = 1;
				$date['mon'] = $date['mon'] + 1;
			}
			break;

		case 'day':
			$date['mday'] = $date['mday'] - $offset;
			break;

		}

	} else {
		// future
		switch ( $type ) {

		case 'year':
		case 'month':
			$_type = ( 'month' == $type ) ? 'mon' : $type;
			$date[ $_type ] = $date[ $_type ] + $offset;
			if ( ( $start_from == 'type' ) && $end_date ) { // end date in the futue
				$date['mday'] = 0; // -1day
			}
			break;

		case 'day': $date['mday'] = $date['mday'] + $offset;
			break;

		}

	}

	// check if day is greater than last day of month
	if ( ( 'year' == $type ) || ( 'month' == $type ) ) {

		// get the total days of the month for this year-month
		$last_day_of_month = date( "t",  mktime( 0, 0, 0, $date['mon'], 1,  $date['year'] ) );

		if ( ( $start_from == 'type' ) && ( 'past' == $future_past ) ) {
			if ( !$end_date ) // start date
				$date['mday'] = $last_day_of_month; // start date
		}

		// check if day bigger than last day of month
		$date['mday'] = ( $date['mday'] > $last_day_of_month ) ? $last_day_of_month : $date['mday'];

	}

	return getdate( mktime( 0, 0, 0, (int) $date['mon'], $date['mday'], $date['year'] ) );
}

See: http://wordpress.org/support/topic/display-pages-with-cumstom-field-date-upcoming