Relative date helper, for 'today' and 'yesterday'

Summary:
Format a date as 'today', 'yesterday', or 'Mar 27 2012'.  Optionally,
the final example can be rendered 'on Mar 27 2012' for things like:

  $excuse =
    'I fell out of a window '.
    phabricator_on_rel_date($time, $me);

Test Plan: Tested in my sandbox!!!!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2035

src/applications/feed/builder/feed/PhabricatorFeedBuilder.php

@@ -59,16 +59,12 @@ final class PhabricatorFeedBuilder {
     require_celerity_resource('phabricator-feed-css');
 
     $last_date = null;
-    $today = phabricator_date(time(), $user);
     foreach ($stories as $story) {
       $story->setHandles($handles);
       $story->setObjects($objects);
       $story->setFramed($this->framed);
 
-      $date = phabricator_date($story->getEpoch(), $user);
+      $date = ucfirst(phabricator_relative_date($story->getEpoch(), $user));
-      if ($date == $today) {
-        $date = 'Today';
-      }
 
       if ($date !== $last_date) {
         if ($last_date !== null) {

src/view/utils/viewutils.php

@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,6 +23,33 @@ function phabricator_date($epoch, $user) {
     'M j Y');
 }
 
+function phabricator_on_relative_date($epoch, $user) {
+  return phabricator_relative_date($epoch, $user, true);
+}
+
+function phabricator_relative_date($epoch, $user, $on = false) {
+  static $today;
+  static $yesterday;
+
+  if (!$today || !$yesterday) {
+    $now = time();
+    $today = phabricator_date($now, $user);
+    $yesterday = phabricator_date($now - 86400, $user);
+  }
+
+  $date = phabricator_date($epoch, $user);
+
+  if ($date === $today) {
+    return 'today';
+  }
+
+  if ($date === $yesterday) {
+    return 'yesterday';
+  }
+
+  return (($on ? 'on ' : '').$date);
+}
+
 function phabricator_time($epoch, $user) {
   return __phabricator_format_local_time(
     $epoch,