diff --git a/Signal/src/ViewControllers/ConversationView/ConversationViewController.m b/Signal/src/ViewControllers/ConversationView/ConversationViewController.m
index 1db0fb6e9..c12916e36 100644
--- a/Signal/src/ViewControllers/ConversationView/ConversationViewController.m
+++ b/Signal/src/ViewControllers/ConversationView/ConversationViewController.m
@@ -3934,8 +3934,8 @@ typedef enum : NSUInteger {
         // No visible messages yet. New Thread.
         return;
     }
-    // MJK FIXME - uncomment and implement
-    //    [OWSReadReceiptManager.sharedManager markAsReadLocallyBeforeSortId:lastVisibleSortId thread:self.thread];
+
+    [OWSReadReceiptManager.sharedManager markAsReadLocallyBeforeSortId:self.lastVisibleSortId thread:self.thread];
 }
 
 - (void)updateGroupModelTo:(TSGroupModel *)newGroupModel successCompletion:(void (^_Nullable)(void))successCompletion
diff --git a/SignalServiceKit/src/Messages/OWSReadReceiptManager.h b/SignalServiceKit/src/Messages/OWSReadReceiptManager.h
index f97e32ab7..e9500db9e 100644
--- a/SignalServiceKit/src/Messages/OWSReadReceiptManager.h
+++ b/SignalServiceKit/src/Messages/OWSReadReceiptManager.h
@@ -69,7 +69,7 @@ extern NSString *const kIncomingMessageMarkedAsReadNotification;
 // This method can be called from any thread.
 - (void)messageWasReadLocally:(TSIncomingMessage *)message;
 
-- (void)markAsReadLocallyBeforeTimestamp:(uint64_t)timestamp thread:(TSThread *)thread;
+- (void)markAsReadLocallyBeforeSortId:(uint64_t)sortId thread:(TSThread *)thread;
 
 #pragma mark - Settings
 
diff --git a/SignalServiceKit/src/Messages/OWSReadReceiptManager.m b/SignalServiceKit/src/Messages/OWSReadReceiptManager.m
index 850c3f9ae..832721f31 100644
--- a/SignalServiceKit/src/Messages/OWSReadReceiptManager.m
+++ b/SignalServiceKit/src/Messages/OWSReadReceiptManager.m
@@ -281,17 +281,17 @@ NSString *const OWSReadReceiptManagerAreReadReceiptsEnabled = @"areReadReceiptsE
 
 #pragma mark - Mark as Read Locally
 
-- (void)markAsReadLocallyBeforeTimestamp:(uint64_t)timestamp thread:(TSThread *)thread
+- (void)markAsReadLocallyBeforeSortId:(uint64_t)sortId thread:(TSThread *)thread
 {
     OWSAssertDebug(thread);
 
     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
         [self.dbConnection readWriteWithBlock:^(YapDatabaseReadWriteTransaction *transaction) {
-            [self markAsReadBeforeTimestamp:timestamp
-                                     thread:thread
-                              readTimestamp:[NSDate ows_millisecondTimeStamp]
-                                   wasLocal:YES
-                                transaction:transaction];
+            [self markAsReadBeforeSortId:sortId
+                                  thread:thread
+                           readTimestamp:[NSDate ows_millisecondTimeStamp]
+                                wasLocal:YES
+                             transaction:transaction];
         }];
     });
 }
@@ -493,27 +493,23 @@ NSString *const OWSReadReceiptManagerAreReadReceiptsEnabled = @"areReadReceiptsE
     // Always re-mark the message as read to ensure any earlier read time is applied to disappearing messages.
     [message markAsReadAtTimestamp:readTimestamp sendReadReceipt:NO transaction:transaction];
 
-    // Also mark any messages appearing earlier in the thread as read.
-    //
-    // Use `timestampForSorting` which reflects local received order, rather than `timestamp`
-    // which reflect sender time.
-    // MJK FIXME - use sortID
-    [self markAsReadBeforeTimestamp:message.timestampForLegacySorting
-                             thread:[message threadWithTransaction:transaction]
-                      readTimestamp:readTimestamp
-                           wasLocal:NO
-                        transaction:transaction];
+    // Also mark any unread messages appearing earlier in the thread as read as well.
+    [self markAsReadBeforeSortId:message.sortId
+                          thread:[message threadWithTransaction:transaction]
+                   readTimestamp:readTimestamp
+                        wasLocal:NO
+                     transaction:transaction];
 }
 
 #pragma mark - Mark As Read
 
-- (void)markAsReadBeforeTimestamp:(uint64_t)timestamp
-                           thread:(TSThread *)thread
-                    readTimestamp:(uint64_t)readTimestamp
-                         wasLocal:(BOOL)wasLocal
-                      transaction:(YapDatabaseReadWriteTransaction *)transaction
+- (void)markAsReadBeforeSortId:(uint64_t)sortId
+                        thread:(TSThread *)thread
+                 readTimestamp:(uint64_t)readTimestamp
+                      wasLocal:(BOOL)wasLocal
+                   transaction:(YapDatabaseReadWriteTransaction *)transaction
 {
-    OWSAssertDebug(timestamp > 0);
+    OWSAssertDebug(sortId > 0);
     OWSAssertDebug(thread);
     OWSAssertDebug(transaction);
 
@@ -537,8 +533,7 @@ NSString *const OWSReadReceiptManagerAreReadReceiptsEnabled = @"areReadReceiptsE
              return;
          }
          id<OWSReadTracking> possiblyRead = (id<OWSReadTracking>)object;
-         // MJK FIXME - use sortId
-         if (possiblyRead.timestampForLegacySorting > timestamp) {
+         if (possiblyRead.sortId > sortId) {
              *stop = YES;
              return;
          }
diff --git a/SignalServiceKit/src/Messages/OWSReadTracking.h b/SignalServiceKit/src/Messages/OWSReadTracking.h
index 04f5d83e6..e3314ea83 100644
--- a/SignalServiceKit/src/Messages/OWSReadTracking.h
+++ b/SignalServiceKit/src/Messages/OWSReadTracking.h
@@ -16,7 +16,7 @@
 @property (nonatomic, readonly, getter=wasRead) BOOL read;
 
 @property (nonatomic, readonly) uint64_t expireStartedAt;
-@property (nonatomic, readonly) uint64_t timestampForLegacySorting;
+@property (nonatomic, readonly) uint64_t sortId;
 @property (nonatomic, readonly) NSString *uniqueThreadId;