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
@@ -2046,6 +2046,7 @@
     'PhabricatorObjectSelectorDialog' => 'view/control/PhabricatorObjectSelectorDialog.php',
     'PhabricatorObjectUsesCredentialsEdgeType' => 'applications/transactions/edges/PhabricatorObjectUsesCredentialsEdgeType.php',
     'PhabricatorOffsetPagedQuery' => 'infrastructure/query/PhabricatorOffsetPagedQuery.php',
+    'PhabricatorOneTimeTriggerClock' => 'infrastructure/daemon/workers/clock/PhabricatorOneTimeTriggerClock.php',
     'PhabricatorOwnerPathQuery' => 'applications/owners/query/PhabricatorOwnerPathQuery.php',
     'PhabricatorOwnersApplication' => 'applications/owners/application/PhabricatorOwnersApplication.php',
     'PhabricatorOwnersConfigOptions' => 'applications/owners/config/PhabricatorOwnersConfigOptions.php',
@@ -2457,6 +2458,7 @@
     'PhabricatorSubscribableInterface' => 'applications/subscriptions/interface/PhabricatorSubscribableInterface.php',
     'PhabricatorSubscribedToObjectEdgeType' => 'applications/transactions/edges/PhabricatorSubscribedToObjectEdgeType.php',
     'PhabricatorSubscribersQuery' => 'applications/subscriptions/query/PhabricatorSubscribersQuery.php',
+    'PhabricatorSubscriptionTriggerClock' => 'infrastructure/daemon/workers/clock/PhabricatorSubscriptionTriggerClock.php',
     'PhabricatorSubscriptionsApplication' => 'applications/subscriptions/application/PhabricatorSubscriptionsApplication.php',
     'PhabricatorSubscriptionsEditController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsEditController.php',
     'PhabricatorSubscriptionsEditor' => 'applications/subscriptions/editor/PhabricatorSubscriptionsEditor.php',
@@ -2520,6 +2522,8 @@
     'PhabricatorTransformedFile' => 'applications/files/storage/PhabricatorTransformedFile.php',
     'PhabricatorTranslation' => 'infrastructure/internationalization/translation/PhabricatorTranslation.php',
     'PhabricatorTranslationsConfigOptions' => 'applications/config/option/PhabricatorTranslationsConfigOptions.php',
+    'PhabricatorTriggerClock' => 'infrastructure/daemon/workers/clock/PhabricatorTriggerClock.php',
+    'PhabricatorTriggerClockTestCase' => 'infrastructure/daemon/workers/clock/__tests__/PhabricatorTriggerClockTestCase.php',
     'PhabricatorTrivialTestCase' => 'infrastructure/testing/__tests__/PhabricatorTrivialTestCase.php',
     'PhabricatorTwitchAuthProvider' => 'applications/auth/provider/PhabricatorTwitchAuthProvider.php',
     'PhabricatorTwitterAuthProvider' => 'applications/auth/provider/PhabricatorTwitterAuthProvider.php',
@@ -5246,6 +5250,7 @@
     'PhabricatorObjectRemarkupRule' => 'PhutilRemarkupRule',
     'PhabricatorObjectUsesCredentialsEdgeType' => 'PhabricatorEdgeType',
     'PhabricatorOffsetPagedQuery' => 'PhabricatorQuery',
+    'PhabricatorOneTimeTriggerClock' => 'PhabricatorTriggerClock',
     'PhabricatorOwnersApplication' => 'PhabricatorApplication',
     'PhabricatorOwnersConfigOptions' => 'PhabricatorApplicationConfigOptions',
     'PhabricatorOwnersController' => 'PhabricatorController',
@@ -5711,6 +5716,7 @@
     'PhabricatorStorageSetupCheck' => 'PhabricatorSetupCheck',
     'PhabricatorSubscribedToObjectEdgeType' => 'PhabricatorEdgeType',
     'PhabricatorSubscribersQuery' => 'PhabricatorQuery',
+    'PhabricatorSubscriptionTriggerClock' => 'PhabricatorTriggerClock',
     'PhabricatorSubscriptionsApplication' => 'PhabricatorApplication',
     'PhabricatorSubscriptionsEditController' => 'PhabricatorController',
     'PhabricatorSubscriptionsEditor' => 'PhabricatorEditor',
@@ -5772,6 +5778,8 @@
     'PhabricatorTransactionsApplication' => 'PhabricatorApplication',
     'PhabricatorTransformedFile' => 'PhabricatorFileDAO',
     'PhabricatorTranslationsConfigOptions' => 'PhabricatorApplicationConfigOptions',
+    'PhabricatorTriggerClock' => 'Phobject',
+    'PhabricatorTriggerClockTestCase' => 'PhabricatorTestCase',
     'PhabricatorTrivialTestCase' => 'PhabricatorTestCase',
     'PhabricatorTwitchAuthProvider' => 'PhabricatorOAuth2AuthProvider',
     'PhabricatorTwitterAuthProvider' => 'PhabricatorOAuth1AuthProvider',
diff --git a/src/infrastructure/daemon/workers/clock/PhabricatorOneTimeTriggerClock.php b/src/infrastructure/daemon/workers/clock/PhabricatorOneTimeTriggerClock.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/daemon/workers/clock/PhabricatorOneTimeTriggerClock.php
@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * Triggers an event exactly once, at a specific epoch time.
+ */
+final class PhabricatorOneTimeTriggerClock
+  extends PhabricatorTriggerClock {
+
+  public function validateProperties(array $properties) {
+    PhutilTypeSpec::checkMap(
+      $properties,
+      array(
+        'epoch' => 'int',
+      ));
+  }
+
+  public function getNextEventEpoch($last_epoch, $is_reschedule) {
+    if ($last_epoch) {
+      return null;
+    }
+
+    return $this->getProperty('epoch');
+  }
+
+}
diff --git a/src/infrastructure/daemon/workers/clock/PhabricatorSubscriptionTriggerClock.php b/src/infrastructure/daemon/workers/clock/PhabricatorSubscriptionTriggerClock.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/daemon/workers/clock/PhabricatorSubscriptionTriggerClock.php
@@ -0,0 +1,81 @@
+<?php
+
+/**
+ * Triggers an event every month on the same day of the month, like the 12th
+ * of the month.
+ *
+ * If a given month does not have such a day (for instance, the clock triggers
+ * on the 30th of each month and the month in question is February, which never
+ * has a 30th day), it will trigger on the last day of the month instead.
+ *
+ * Choosing this strategy for subscriptions is predictable (it's easy to
+ * anticipate when a subscription period will end) and fair (billing
+ * periods always have nearly equal length). It also spreads subscriptions
+ * out evenly. If there are issues with billing, this provides an opportunity
+ * for them to be corrected after only a few customers are affected, instead of
+ * (for example) having every subscription fail all at once on the 1st of the
+ * month.
+ */
+final class PhabricatorSubscriptionTriggerClock
+  extends PhabricatorTriggerClock {
+
+  public function validateProperties(array $properties) {
+    PhutilTypeSpec::checkMap(
+      $properties,
+      array(
+        'start' => 'int',
+      ));
+  }
+
+  public function getNextEventEpoch($last_epoch, $is_reschedule) {
+    $start_epoch = $this->getProperty('start');
+    if (!$last_epoch) {
+      $last_epoch = $start_epoch;
+    }
+
+    // Constructing DateTime objects like this implies UTC, so we don't need
+    // to set that explicitly.
+    $start = new DateTime('@'.$start_epoch);
+    $last = new DateTime('@'.$last_epoch);
+
+    $year = (int)$last->format('Y');
+    $month = (int)$last->format('n');
+
+    // Note that we're getting the day of the month from the start date, not
+    // from the last event date. This lets us schedule on March 31 after moving
+    // the date back to Feb 28.
+    $day = (int)$start->format('j');
+
+    // We trigger at the same time of day as the original event. Generally,
+    // this means that you should get invoiced at a reasonable local time in
+    // most cases, unless you subscribed at 1AM or something.
+    $hms = $start->format('G:i:s');
+
+    // Increment the month by 1.
+    $month = $month + 1;
+
+    // If we ran off the end of the calendar, set the month back to January
+    // and increment the year by 1.
+    if ($month > 12) {
+      $month = 1;
+      $year = $year + 1;
+    }
+
+    // Now, move the day backward until it falls in the correct month. If we
+    // pass an invalid date like "2014-2-31", it will internally be parsed
+    // as though we had passed "2014-3-3".
+    while (true) {
+      $next = new DateTime("{$year}-{$month}-{$day} {$hms} UTC");
+      if ($next->format('n') == $month) {
+        // The month didn't get corrected forward, so we're all set.
+        break;
+      } else {
+        // The month did get corrected forward, so back off a day.
+        $day--;
+      }
+    }
+
+    return (int)$next->format('U');
+  }
+
+}
diff --git a/src/infrastructure/daemon/workers/clock/PhabricatorTriggerClock.php b/src/infrastructure/daemon/workers/clock/PhabricatorTriggerClock.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/daemon/workers/clock/PhabricatorTriggerClock.php
@@ -0,0 +1,74 @@
+<?php
+
+/**
+ * A trigger clock implements scheduling rules for an event.
+ *
+ * Two examples of triggered events are a subscription which bills on the 12th
+ * of every month, or a meeting reminder which sends an email 15 minutes before
+ * an event. A trigger clock contains the logic to figure out exactly when
+ * those times are.
+ *
+ * For example, it might schedule an event every hour, or every Thursday, or on
+ * the 15th of every month at 3PM, or only at a specific time.
+ */
+abstract class PhabricatorTriggerClock extends Phobject {
+
+  private $properties;
+
+  public function __construct(array $properties) {
+    $this->validateProperties($properties);
+    $this->properties = $properties;
+  }
+
+  public function getProperties() {
+    return $this->properties;
+  }
+
+  public function getProperty($key, $default = null) {
+    return idx($this->properties, $key, $default);
+  }
+
+
+  /**
+   * Validate clock configuration.
+   *
+   * @param map<string, wild> Map of clock properties.
+   * @return void
+   */
+  abstract public function validateProperties(array $properties);
+
+
+  /**
+   * Get the next occurrence of this event.
+   *
+   * This method takes two parameters: the last time this event occurred (or
+   * null if it has never triggered before) and a flag distinguishing between
+   * a normal reschedule (after a successful trigger) or an update because of
+   * a trigger change.
+   *
+   * If this event does not occur again, return `null` to stop it from being
+   * rescheduled. For example, a meeting reminder may be sent only once before
+   * the meeting.
+   *
+   * If this event does occur again, return the epoch timestamp of the next
+   * occurrence.
+   *
+   * When performing routine reschedules, the event must move forward in time:
+   * any timestamp you return must be later than the last event. For instance,
+   * if this event triggers an invoice, the next invoice date must be after
+   * the previous invoice date. This prevents an event from looping more than
+   * once per second.
+   *
+   * In contrast, after an update (not a routine reschedule), the next event
+   * may be scheduled at any time. For example, if a meeting is moved from next
+   * week to 3 minutes from now, the clock may reschedule the notification to
+   * occur 12 minutes ago. This will cause it to execute immediately.
+   *
+   * @param int|null Last time the event occurred, or null if it has never
+   *   triggered before.
+   * @param bool True if this is a reschedule after a successful trigger.
+   * @return int|null Next event, or null to decline to reschedule.
+   */
+  abstract public function getNextEventEpoch($last_epoch, $is_reschedule);
+
+}
diff --git a/src/infrastructure/daemon/workers/clock/__tests__/PhabricatorTriggerClockTestCase.php b/src/infrastructure/daemon/workers/clock/__tests__/PhabricatorTriggerClockTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/daemon/workers/clock/__tests__/PhabricatorTriggerClockTestCase.php
@@ -0,0 +1,85 @@
+<?php
+
+final class PhabricatorTriggerClockTestCase extends PhabricatorTestCase {
+
+  public function testOneTimeTriggerClock() {
+    $now = PhabricatorTime::getNow();
+
+    $clock = new PhabricatorOneTimeTriggerClock(
+      array(
+        'epoch' => $now,
+      ));
+
+    $this->assertEqual(
+      $now,
+      $clock->getNextEventEpoch(null, false),
+      pht('Should trigger at specified epoch.'));
+
+    $this->assertEqual(
+      null,
+      $clock->getNextEventEpoch(1, false),
+      pht('Should trigger only once.'));
+  }
+
+  public function testSubscriptionTriggerClock() {
+    $start = strtotime('2014-01-31 2:34:56 UTC');
+
+    $clock = new PhabricatorSubscriptionTriggerClock(
+      array(
+        'start' => $start,
+      ));
+
+    $expect_list = array(
+      // This should be moved to the 28th of February.
+      '2014-02-28 2:34:56',
+
+      // In March, which has 31 days, it should move back to the 31st.
+      '2014-03-31 2:34:56',
+
+      // On months with only 30 days, it should occur on the 30th.
+      '2014-04-30 2:34:56',
+      '2014-05-31 2:34:56',
+      '2014-06-30 2:34:56',
+      '2014-07-31 2:34:56',
+      '2014-08-31 2:34:56',
+      '2014-09-30 2:34:56',
+      '2014-10-31 2:34:56',
+      '2014-11-30 2:34:56',
+      '2014-12-31 2:34:56',
+
+      // After billing on Dec 31 2014, it should wrap around to Jan 31 2015.
+      '2015-01-31 2:34:56',
+      '2015-02-28 2:34:56',
+      '2015-03-31 2:34:56',
+      '2015-04-30 2:34:56',
+      '2015-05-31 2:34:56',
+      '2015-06-30 2:34:56',
+      '2015-07-31 2:34:56',
+      '2015-08-31 2:34:56',
+      '2015-09-30 2:34:56',
+      '2015-10-31 2:34:56',
+      '2015-11-30 2:34:56',
+      '2015-12-31 2:34:56',
+      '2016-01-31 2:34:56',
+
+      // Finally, this should bill on leap day in 2016.
+      '2016-02-29 2:34:56',
+      '2016-03-31 2:34:56',
+    );
+
+    $last_epoch = null;
+    foreach ($expect_list as $cycle => $expect) {
+      $next_epoch = $clock->getNextEventEpoch(
+        $last_epoch,
+        ($last_epoch !== null));
+
+      $this->assertEqual(
+        $expect,
+        id(new DateTime('@'.$next_epoch))->format('Y-m-d g:i:s'),
+        pht('Billing cycle %s.', $cycle));
+
+      $last_epoch = $next_epoch;
+    }
+  }
+
+}