And added a test for it.

src/NzbDrone.Core.Test/Download/DownloadClientTests/TransmissionTests/TransmissionFixture.cs

@@ -371,5 +371,16 @@ namespace NzbDrone.Core.Test.Download.DownloadClientTests.TransmissionTests
 
             Subject.Test();
         }
+
+        [TestCase(-1)] // Infinite/Unknown
+        [TestCase(-2)] // Magnet Downloading
+        public void should_ignore_negative_eta(int eta)
+        {
+            _completed.Eta = eta;
+
+            PrepareClientToReturnCompletedItem();
+            var item = Subject.GetItems().Single();
+            item.RemainingTime.Should().NotHaveValue();
+        }
     }
 }

src/NzbDrone.Core/Download/Clients/Transmission/Transmission.cs

@@ -117,7 +117,7 @@ namespace NzbDrone.Core.Download.Clients.Transmission
                 item.OutputPath = outputPath + torrent.Name;
                 item.TotalSize = torrent.TotalSize;
                 item.RemainingSize = torrent.LeftUntilDone;
-                if (torrent.Eta != -1)
+                if (torrent.Eta >= 0)
                 {
                     item.RemainingTime = TimeSpan.FromSeconds(torrent.Eta);
                 }