diff --git a/Manatee.Trello/ReadOnlyCardCollection.cs b/Manatee.Trello/ReadOnlyCardCollection.cs
index 405c0c60..896a2a24 100644
--- a/Manatee.Trello/ReadOnlyCardCollection.cs
+++ b/Manatee.Trello/ReadOnlyCardCollection.cs
@@ -77,9 +77,9 @@ public void Filter(CardFilter filter)
 		public void Filter(DateTime? start, DateTime? end)
 		{
 			if (start != null)
-				AdditionalParameters["since"] = start;
+				AdditionalParameters["since"] = start.Value.ToUniversalTime().ToString("O");
 			if (end != null)
-				AdditionalParameters["before"] = end;
+				AdditionalParameters["before"] = end.Value.ToUniversalTime().ToString("O");
 		}
 
 		internal sealed override async Task PerformRefresh(bool force, CancellationToken ct)
@@ -151,4 +151,4 @@ void IHandle<EntityDeletedEvent<IJsonCard>>.Handle(EntityDeletedEvent<IJsonCard>
 			Items.Remove(item);
 		}
 	}
-}
\ No newline at end of file
+}