diff --git a/classes/ComicPress.inc b/classes/ComicPress.inc
index cadcbc1..ba7a956 100644
--- a/classes/ComicPress.inc
+++ b/classes/ComicPress.inc
@@ -294,3 +294,6 @@ class ComicPress {
 		return false;
 	}
 }
+
+class ComicPressException extends Exception {}
+
diff --git a/classes/ComicPressTagBuilder.inc b/classes/ComicPressTagBuilder.inc
index 4f9328c..54606d8 100644
--- a/classes/ComicPressTagBuilder.inc
+++ b/classes/ComicPressTagBuilder.inc
@@ -35,16 +35,20 @@ class ComicPressTagBuilder {
 	}
 
 	public function __call($method, $arguments) {
+		$ok = false;
+
 		switch ($method) {
 			case 'next':
 			case 'previous':
 			case 'first':
 			case 'last':
 				$this->post = call_user_func(array($this->dbi, "get_${method}_post"), $this->storyline->build_from_restrictions($this->restrictions), $this->parent_post);
+				$ok = true;
 				break;
 			case 'in':
 				if (!isset($arguments[0])) { throw new Exception('Need to specify a category');	}
 				$this->restrictions[] = array('child_of', $arguments[0]);
+				$ok = true;
 				break;
 			case 'id':
 				return $this->post->ID;
@@ -52,6 +56,11 @@ class ComicPressTagBuilder {
 				return $this->post->post_title;
 			case 'timestamp':
 				return strtotime($this->post->post_date);
+			case 'date':
+				if (isset($arguments[0])) {
+					return date($arguments[0], strtotime($this->post->post_date));
+				}
+				break;
 			case 'permalink':
 				return get_permalink($this->post->ID);
 			case 'post':
@@ -63,11 +72,16 @@ class ComicPressTagBuilder {
 						$new_method = array_shift($method_info);
 						call_user_func_array(array($this, $new_method), $method_info);
 					}
+					$ok = true;
 				}
 				break;
 		}
 
-		return $this;
+		if ($ok) {
+			return $this;
+		} else {
+			throw new ComicPressException("${method} isn't allowed at this point");
+		}
 	}
 
 	public function parse_method($method_name) {
diff --git a/test/ComicPressTagBuilderTest.php b/test/ComicPressTagBuilderTest.php
index 57eb308..987bb3b 100644
--- a/test/ComicPressTagBuilderTest.php
+++ b/test/ComicPressTagBuilderTest.php
@@ -102,7 +102,8 @@ class ComicPressTagBuilderTest extends PHPUnit_Framework_TestCase {
 				'post_title' => 'Post title',
 				'post_date' => '2010-01-01',
 				'guid' => 'the-slug',
-			))
+			)),
+			array(array('date', 'Ymd'), '20100101'),
 		);
 	}
 
@@ -128,7 +129,52 @@ class ComicPressTagBuilderTest extends PHPUnit_Framework_TestCase {
 		$core = new ComicPressTagBuilderFactory($dbi);
 		$core = $core->first_in_1();
 
-		$this->assertEquals($expected_result, $core->{$info_method}());
+		if (is_array($info_method)) {
+			$method = array_shift($info_method);
+			$this->assertEquals($expected_result, call_user_func_array(array($core, $method), $info_method));
+		} else {
+			$this->assertEquals($expected_result, $core->{$info_method}());
+		}
+	}
+
+	function providerTestStorylineBuilderExceptions() {
+		return array(
+			array(array('bad')),
+			array(array(array('in', 1), 'bad')),
+			array(array(array('in', 1), 'date')),
+		);
+	}
+
+	/**
+	 * @expectedException ComicPressException
+	 * @dataProvider providerTestStorylineBuilderExceptions
+	 */
+	function testStorylineBuilderExceptions($calls) {
+		$target_post = (object)array(
+			'ID' => 1,
+			'post_title' => 'Post title',
+			'post_date' => '2010-01-01',
+			'guid' => 'the-slug',
+		);
+
+		wp_insert_post($target_post);
+
+		$dbi = $this->getMock('ComicPressDBInterface', array('get_first_post'));
+		$dbi->expects($this->any())->method('get_first_post')->will($this->returnValue($target_post));
+
+		$storyline = new ComicPressStoryline();
+		$storyline->set_flattened_storyline('0/1');
+
+		$core = new ComicPressTagBuilderFactory($dbi);
+
+		foreach ($calls as $call) {
+			if (is_array($call)) {
+				$method = array_shift($call);
+				$core = call_user_func_array(array($core, $method), $call);
+			} else {
+				$core = $core->{$call}();
+			}
+		}
 	}
 
 	function providerTestMethodParser() {