Use TimeUnits for sort key calculations

The unspecified units were confusing when writing tests around these
values, so be explicit about units in variable names and quantities
for conversion.

Use Ints.checkedCast so we fail fast when inserting a change 4000+
years in the future rather than having it not show up in search
results.

Correct the sort key epoch time, which likely had a timezone error
when initially calculated:

$ python -c 'import datetime; print datetime.datetime.fromtimestamp(1222819200L)'
2008-09-30 17:00:00

This mistake does not affect any data; the comment in the code was
just confusingly incorrect.

Change-Id: I6d8bcf4985e91c3d115002db42642049da8c708f
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/ChangeUtil.java b/gerrit-server/src/main/java/com/google/gerrit/server/ChangeUtil.java
index 560bee3..3c460b8 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/ChangeUtil.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/ChangeUtil.java
@@ -15,8 +15,12 @@
 package com.google.gerrit.server;
 
 import static com.google.gerrit.server.change.PatchSetInserter.ValidatePolicy.RECEIVE_COMMITS;
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static java.util.concurrent.TimeUnit.MINUTES;
+import static java.util.concurrent.TimeUnit.SECONDS;
 
 import com.google.common.annotations.VisibleForTesting;
+import com.google.common.primitives.Ints;
 import com.google.gerrit.common.ChangeHooks;
 import com.google.gerrit.common.errors.EmailException;
 import com.google.gerrit.reviewdb.client.Change;
@@ -80,8 +84,14 @@
 import java.util.regex.Matcher;
 
 public class ChangeUtil {
+  /**
+   * Epoch for sort key calculations, Tue Sep 30 2008 17:00:00.
+   * <p>
+   * We overrun approximately 4,083 years later, so ~6092.
+   */
   @VisibleForTesting
-  public static final long SORT_KEY_EPOCH = 1222819200L; // Oct 1 2008 00:00
+  public static final long SORT_KEY_EPOCH_MINS =
+      MINUTES.convert(1222819200L, SECONDS);
 
   private static final Object uuidLock = new Object();
   private static final int SEED = 0x2418e6f9;
@@ -465,14 +475,12 @@
     db.patchSets().delete(Collections.singleton(patch));
   }
 
-  public static String sortKey(long lastUpdated, int id){
-    // The encoding uses minutes since Wed Oct 1 00:00:00 2008 UTC.
-    // We overrun approximately 4,085 years later, so ~6093.
-    //
-    final long lastUpdatedOn = (lastUpdated / 1000L) - SORT_KEY_EPOCH;
-    final StringBuilder r = new StringBuilder(16);
+  public static String sortKey(long lastUpdatedMs, int id){
+    long lastUpdatedMins = MINUTES.convert(lastUpdatedMs, MILLISECONDS);
+    long minsSinceEpoch = lastUpdatedMins - SORT_KEY_EPOCH_MINS;
+    StringBuilder r = new StringBuilder(16);
     r.setLength(16);
-    formatHexInt(r, 0, (int) (lastUpdatedOn / 60));
+    formatHexInt(r, 0, Ints.checkedCast(minsSinceEpoch));
     formatHexInt(r, 8, id);
     return r.toString();
   }
@@ -484,10 +492,10 @@
     return Long.parseLong(sortKey, 16);
   }
 
-  public static void computeSortKey(final Change c) {
-    long lastUpdated = c.getLastUpdatedOn().getTime();
+  public static void computeSortKey(Change c) {
+    long lastUpdatedMs = c.getLastUpdatedOn().getTime();
     int id = c.getId().get();
-    c.setSortKey(sortKey(lastUpdated, id));
+    c.setSortKey(sortKey(lastUpdatedMs, id));
   }
 
   public static PatchSet.Id nextPatchSetId(Map<String, Ref> allRefs,
diff --git a/gerrit-server/src/test/java/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java b/gerrit-server/src/test/java/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java
index 9829c98..e57ceab 100644
--- a/gerrit-server/src/test/java/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java
+++ b/gerrit-server/src/test/java/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java
@@ -14,6 +14,9 @@
 
 package com.google.gerrit.server.query.change;
 
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static java.util.concurrent.TimeUnit.MINUTES;
+import static java.util.concurrent.TimeUnit.DAYS;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 
@@ -85,7 +88,9 @@
   protected Account.Id userId;
   protected CurrentUser user;
   protected int clockStepMs = 1;
-  private long ts = ChangeUtil.SORT_KEY_EPOCH * 1000;
+  private long clockMs =
+      MILLISECONDS.convert(ChangeUtil.SORT_KEY_EPOCH_MINS, MINUTES)
+      + MILLISECONDS.convert(60, DAYS);
 
   protected abstract Injector createInjector();
 
@@ -416,8 +421,8 @@
 
     Change change = new Change(new Change.Key(key), id, ownerId,
         new Branch.NameKey(project, branch));
-    change.setLastUpdatedOn(new Timestamp(ts));
-    ts += clockStepMs;
+    change.setLastUpdatedOn(new Timestamp(clockMs));
+    clockMs += clockStepMs;
     return changeFactory.create(
         projectControlFactory.controlFor(project,
           userFactory.create(ownerId)).controlFor(change).getRefControl(),