Source

File: wp-includes/class-wp-date-query.php

public function build_mysql_datetime( $datetime, $default_to_max = false ) {
		if ( ! is_array( $datetime ) ) {

			/*
			 * Try to parse some common date formats, so we can detect
			 * the level of precision and support the 'inclusive' parameter.
			 */
			if ( preg_match( '/^(\d{4})$/', $datetime, $matches ) ) {
				// Y
				$datetime = array(
					'year' => intval( $matches[1] ),
				);

			} elseif ( preg_match( '/^(\d{4})\-(\d{2})$/', $datetime, $matches ) ) {
				// Y-m
				$datetime = array(
					'year'  => intval( $matches[1] ),
					'month' => intval( $matches[2] ),
				);

			} elseif ( preg_match( '/^(\d{4})\-(\d{2})\-(\d{2})$/', $datetime, $matches ) ) {
				// Y-m-d
				$datetime = array(
					'year'  => intval( $matches[1] ),
					'month' => intval( $matches[2] ),
					'day'   => intval( $matches[3] ),
				);

			} elseif ( preg_match( '/^(\d{4})\-(\d{2})\-(\d{2}) (\d{2}):(\d{2})$/', $datetime, $matches ) ) {
				// Y-m-d H:i
				$datetime = array(
					'year'   => intval( $matches[1] ),
					'month'  => intval( $matches[2] ),
					'day'    => intval( $matches[3] ),
					'hour'   => intval( $matches[4] ),
					'minute' => intval( $matches[5] ),
				);
			}

			// If no match is found, we don't support default_to_max.
			if ( ! is_array( $datetime ) ) {
				$wp_timezone = wp_timezone();

				// Assume local timezone if not provided.
				$dt = date_create( $datetime, $wp_timezone );

				if ( false === $dt ) {
					return gmdate( 'Y-m-d H:i:s', false );
				}

				return $dt->setTimezone( $wp_timezone )->format( 'Y-m-d H:i:s' );
			}
		}

		$datetime = array_map( 'absint', $datetime );

		if ( ! isset( $datetime['year'] ) ) {
			$datetime['year'] = current_time( 'Y' );
		}

		if ( ! isset( $datetime['month'] ) ) {
			$datetime['month'] = ( $default_to_max ) ? 12 : 1;
		}

		if ( ! isset( $datetime['day'] ) ) {
			$datetime['day'] = ( $default_to_max ) ? (int) gmdate( 't', mktime( 0, 0, 0, $datetime['month'], 1, $datetime['year'] ) ) : 1;
		}

		if ( ! isset( $datetime['hour'] ) ) {
			$datetime['hour'] = ( $default_to_max ) ? 23 : 0;
		}

		if ( ! isset( $datetime['minute'] ) ) {
			$datetime['minute'] = ( $default_to_max ) ? 59 : 0;
		}

		if ( ! isset( $datetime['second'] ) ) {
			$datetime['second'] = ( $default_to_max ) ? 59 : 0;
		}

		return sprintf( '%04d-%02d-%02d %02d:%02d:%02d', $datetime['year'], $datetime['month'], $datetime['day'], $datetime['hour'], $datetime['minute'], $datetime['second'] );
	}