Index: src/__phutil_library_map__.php
===================================================================
--- src/__phutil_library_map__.php
+++ src/__phutil_library_map__.php
@@ -191,6 +191,8 @@
     'PhutilLipsumContextFreeGrammar' => 'grammar/PhutilLipsumContextFreeGrammar.php',
     'PhutilLock' => 'filesystem/PhutilLock.php',
     'PhutilLockException' => 'filesystem/PhutilLockException.php',
+    'PhutilLunarPhase' => 'utils/PhutilLunarPhase.php',
+    'PhutilLunarPhaseTestCase' => 'utils/__tests__/PhutilLunarPhaseTestCase.php',
     'PhutilMarkupEngine' => 'markup/PhutilMarkupEngine.php',
     'PhutilMarkupTestCase' => 'markup/__tests__/PhutilMarkupTestCase.php',
     'PhutilMetricsChannel' => 'channel/PhutilMetricsChannel.php',
@@ -548,6 +550,7 @@
     'PhutilLexerSyntaxHighlighter' => 'PhutilSyntaxHighlighter',
     'PhutilLipsumContextFreeGrammar' => 'PhutilContextFreeGrammar',
     'PhutilLockException' => 'Exception',
+    'PhutilLunarPhaseTestCase' => 'PhutilTestCase',
     'PhutilMarkupTestCase' => 'PhutilTestCase',
     'PhutilMetricsChannel' => 'PhutilChannelChannel',
     'PhutilMissingSymbolException' => 'Exception',
Index: src/utils/PhutilLunarPhase.php
===================================================================
--- /dev/null
+++ src/utils/PhutilLunarPhase.php
@@ -0,0 +1,43 @@
+<?php
+
+/**
+ * Compute the phase of the terran moon for a given epoch.
+ *
+ *   $moon = new PhutilLunarPhase(time());
+ *   $moon->isFull();
+ */
+final class PhutilLunarPhase {
+
+  private $epoch;
+
+  public function __construct($epoch) {
+    $this->epoch = $epoch;
+  }
+
+  private function getPhase() {
+    // (Aug 11, 1999) A new moon.
+    $new_moon_epoch = 934354800;
+
+    // Number of seconds in the lunar phase.
+    $phase_length = 2551442.8768992;
+
+    return fmod($this->epoch - $new_moon_epoch, $phase_length) / $phase_length;
+  }
+
+  public function isFull() {
+    return abs($this->getPhase() - 0.5) < 0.02;
+  }
+
+  public function isNew() {
+    return $this->getPhase() < 0.02 || $this->getPhase() > 0.98;
+  }
+
+  public function isWaxing() {
+    return $this->getPhase() < 0.5;
+  }
+
+  public function isWaning() {
+    return $this->getPhase() > 0.5;
+  }
+
+}
Index: src/utils/__tests__/PhutilLunarPhaseTestCase.php
===================================================================
--- /dev/null
+++ src/utils/__tests__/PhutilLunarPhaseTestCase.php
@@ -0,0 +1,59 @@
+<?php
+
+final class PhutilLunarPhaseTestCase extends PhutilTestCase {
+
+  public function testLunarPhases() {
+
+    // Aug 11, 1999
+    $moon = new PhutilLunarPhase(934354800);
+    $this->assertEqual(false, $moon->isFull());
+    $this->assertEqual(true, $moon->isNew());
+    $this->assertEqual(true, $moon->isWaxing());
+    $this->assertEqual(false, $moon->isWaning());
+
+    // May 22, 2005
+    $moon = new PhutilLunarPhase(1116745200);
+    $this->assertEqual(true, $moon->isFull());
+    $this->assertEqual(false, $moon->isNew());
+    $this->assertEqual(true, $moon->isWaxing());
+    $this->assertEqual(false, $moon->isWaning());
+
+    // May 23, 2005
+    $moon = new PhutilLunarPhase(1116831600);
+    $this->assertEqual(true, $moon->isFull());
+    $this->assertEqual(false, $moon->isNew());
+    $this->assertEqual(false, $moon->isWaxing());
+    $this->assertEqual(true, $moon->isWaning());
+
+    // May 30, 2005
+    $moon = new PhutilLunarPhase(1117436400);
+    $this->assertEqual(false, $moon->isFull());
+    $this->assertEqual(false, $moon->isNew());
+    $this->assertEqual(false, $moon->isWaxing());
+    $this->assertEqual(true, $moon->isWaning());
+
+    // June 05, 2005
+    $moon = new PhutilLunarPhase(1117954800);
+    $this->assertEqual(false, $moon->isFull());
+    $this->assertEqual(false, $moon->isNew());
+    $this->assertEqual(false, $moon->isWaxing());
+    $this->assertEqual(true, $moon->isWaning());
+
+    // June 06, 2005
+    $moon = new PhutilLunarPhase(1118041200);
+    $this->assertEqual(false, $moon->isFull());
+    $this->assertEqual(true, $moon->isNew());
+    $this->assertEqual(false, $moon->isWaxing());
+    $this->assertEqual(true, $moon->isWaning());
+
+    // Oct 4, 2013
+    $moon = new PhutilLunarPhase(1380897327);
+    $this->assertEqual(false, $moon->isFull());
+    $this->assertEqual(true, $moon->isNew());
+    $this->assertEqual(true, $moon->isWaxing());
+    $this->assertEqual(false, $moon->isWaning());
+
+  }
+
+
+}