diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -135,6 +135,7 @@
     'PhutilCalendarAbsoluteDateTime' => 'parser/calendar/data/PhutilCalendarAbsoluteDateTime.php',
     'PhutilCalendarContainerNode' => 'parser/calendar/data/PhutilCalendarContainerNode.php',
     'PhutilCalendarDateTime' => 'parser/calendar/data/PhutilCalendarDateTime.php',
+    'PhutilCalendarDateTimeTestCase' => 'parser/calendar/data/__tests__/PhutilCalendarDateTimeTestCase.php',
     'PhutilCalendarDocumentNode' => 'parser/calendar/data/PhutilCalendarDocumentNode.php',
     'PhutilCalendarDuration' => 'parser/calendar/data/PhutilCalendarDuration.php',
     'PhutilCalendarEventNode' => 'parser/calendar/data/PhutilCalendarEventNode.php',
@@ -732,6 +733,7 @@
     'PhutilCalendarAbsoluteDateTime' => 'PhutilCalendarDateTime',
     'PhutilCalendarContainerNode' => 'PhutilCalendarNode',
     'PhutilCalendarDateTime' => 'Phobject',
+    'PhutilCalendarDateTimeTestCase' => 'PhutilTestCase',
     'PhutilCalendarDocumentNode' => 'PhutilCalendarContainerNode',
     'PhutilCalendarDuration' => 'Phobject',
     'PhutilCalendarEventNode' => 'PhutilCalendarContainerNode',
diff --git a/src/parser/calendar/data/PhutilCalendarDateTime.php b/src/parser/calendar/data/PhutilCalendarDateTime.php
--- a/src/parser/calendar/data/PhutilCalendarDateTime.php
+++ b/src/parser/calendar/data/PhutilCalendarDateTime.php
@@ -46,16 +46,9 @@
     }
   }
 
-  public function newAbsoluteDateTime() {
-    $epoch = $this->getEpoch();
-    $timezone = nonempty($this->getTimezone(), 'UTC');
-    return PhutilCalendarAbsoluteDateTime::newFromEpoch($epoch, $timezone)
-      ->setIsAllDay($this->getIsAllDay())
-      ->setViewerTimezone($this->getViewerTimezone());
-  }
-
   abstract public function newPHPDateTimeZone();
   abstract public function newPHPDateTime();
+  abstract public function newAbsoluteDateTime();
 
   abstract public function getTimezone();
 }
diff --git a/src/parser/calendar/data/PhutilCalendarProxyDateTime.php b/src/parser/calendar/data/PhutilCalendarProxyDateTime.php
--- a/src/parser/calendar/data/PhutilCalendarProxyDateTime.php
+++ b/src/parser/calendar/data/PhutilCalendarProxyDateTime.php
@@ -14,6 +14,10 @@
     return $this->proxy;
   }
 
+  public function __clone() {
+    $this->proxy = clone $this->proxy;
+  }
+
   public function setViewerTimezone($timezone) {
     $this->getProxy()->setViewerTimezone($timezone);
     return $this;
diff --git a/src/parser/calendar/data/PhutilCalendarRelativeDateTime.php b/src/parser/calendar/data/PhutilCalendarRelativeDateTime.php
--- a/src/parser/calendar/data/PhutilCalendarRelativeDateTime.php
+++ b/src/parser/calendar/data/PhutilCalendarRelativeDateTime.php
@@ -50,4 +50,22 @@
     return $datetime;
   }
 
+  public function newAbsoluteDateTime() {
+    $clone = clone $this;
+    $clone->setViewerTimezone(null);
+
+    $datetime = $clone->newPHPDateTime();
+
+    return id(new PhutilCalendarAbsoluteDateTime())
+      ->setYear((int)$datetime->format('Y'))
+      ->setMonth((int)$datetime->format('m'))
+      ->setDay((int)$datetime->format('d'))
+      ->setHour((int)$datetime->format('H'))
+      ->setMinute((int)$datetime->format('i'))
+      ->setSecond((int)$datetime->format('s'))
+      ->setIsAllDay($clone->getIsAllDay())
+      ->setTimezone($clone->getTimezone())
+      ->setViewerTimezone($this->getViewerTimezone());
+  }
+
 }
diff --git a/src/parser/calendar/data/__tests__/PhutilCalendarDateTimeTestCase.php b/src/parser/calendar/data/__tests__/PhutilCalendarDateTimeTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/parser/calendar/data/__tests__/PhutilCalendarDateTimeTestCase.php
@@ -0,0 +1,29 @@
+<?php
+
+final class PhutilCalendarDateTimeTestCase extends PhutilTestCase {
+
+  public function testDateTimeDuration() {
+    $start = PhutilCalendarAbsoluteDateTime::newFromISO8601('20161128T090000Z')
+      ->setTimezone('America/Los_Angeles')
+      ->setViewerTimezone('America/Chicago')
+      ->setIsAllDay(true);
+
+    $this->assertEqual(
+      '20161128',
+      $start->getISO8601());
+
+    $end = $start
+      ->newAbsoluteDateTime()
+      ->setHour(0)
+      ->setMinute(0)
+      ->setSecond(0)
+      ->newRelativeDateTime('P1D')
+      ->newAbsoluteDateTime();
+
+    $this->assertEqual(
+      '20161129',
+      $end->getISO8601());
+  }
+
+
+}