Source

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

public function build_time_query( $column, $compare, $hour = null, $minute = null, $second = null ) {
		global $wpdb;

		// Have to have at least one.
		if ( ! isset( $hour ) && ! isset( $minute ) && ! isset( $second ) ) {
			return false;
		}

		// Complex combined queries aren't supported for multi-value queries.
		if ( in_array( $compare, array( 'IN', 'NOT IN', 'BETWEEN', 'NOT BETWEEN' ), true ) ) {
			$return = array();

			$value = $this->build_value( $compare, $hour );
			if ( false !== $value ) {
				$return[] = "HOUR( $column ) $compare $value";
			}

			$value = $this->build_value( $compare, $minute );
			if ( false !== $value ) {
				$return[] = "MINUTE( $column ) $compare $value";
			}

			$value = $this->build_value( $compare, $second );
			if ( false !== $value ) {
				$return[] = "SECOND( $column ) $compare $value";
			}

			return implode( ' AND ', $return );
		}

		// Cases where just one unit is set.
		if ( isset( $hour ) && ! isset( $minute ) && ! isset( $second ) ) {
			$value = $this->build_value( $compare, $hour );
			if ( false !== $value ) {
				return "HOUR( $column ) $compare $value";
			}
		} elseif ( ! isset( $hour ) && isset( $minute ) && ! isset( $second ) ) {
			$value = $this->build_value( $compare, $minute );
			if ( false !== $value ) {
				return "MINUTE( $column ) $compare $value";
			}
		} elseif ( ! isset( $hour ) && ! isset( $minute ) && isset( $second ) ) {
			$value = $this->build_value( $compare, $second );
			if ( false !== $value ) {
				return "SECOND( $column ) $compare $value";
			}
		}

		// Single units were already handled. Since hour & second isn't allowed, minute must to be set.
		if ( ! isset( $minute ) ) {
			return false;
		}

		$format = '';
		$time   = '';

		// Hour.
		if ( null !== $hour ) {
			$format .= '%H.';
			$time   .= sprintf( '%02d', $hour ) . '.';
		} else {
			$format .= '0.';
			$time   .= '0.';
		}

		// Minute.
		$format .= '%i';
		$time   .= sprintf( '%02d', $minute );

		if ( isset( $second ) ) {
			$format .= '%s';
			$time   .= sprintf( '%02d', $second );
		}

		return $wpdb->prepare( "DATE_FORMAT( $column, %s ) $compare %f", $format, $time );
	}