diff --git a/rx/rx.time-lite.d.ts b/rx/rx.time-lite.d.ts
index a5b102827..7a7d8f46f 100644
--- a/rx/rx.time-lite.d.ts
+++ b/rx/rx.time-lite.d.ts
@@ -49,8 +49,8 @@ declare module Rx {
 	interface ObservableStatic {
 		interval(period: number, scheduler?: IScheduler): Observable<number>;
 		interval(dutTime: number, period: number, scheduler?: IScheduler): Observable<number>;
-		timer(dueTime: number, period: number, scheduler: IScheduler): Observable<number>;
-		timer(dueTime: number, scheduler: IScheduler): Observable<number>;
+		timer(dueTime: number, period: number, scheduler?: IScheduler): Observable<number>;
+		timer(dueTime: number, scheduler?: IScheduler): Observable<number>;
 		generateWithRelativeTime<TState, TResult>(
 			initialState: TState,
 			condition: (state: TState) => boolean,
diff --git a/rx/rx.time.d.ts b/rx/rx.time.d.ts
index 6efd7d9cf..2cdb1d56a 100644
--- a/rx/rx.time.d.ts
+++ b/rx/rx.time.d.ts
@@ -17,8 +17,8 @@ declare module Rx {
 	}
 
 	interface ObservableStatic {
-		timer(dueTime: Date, period: number, scheduler: IScheduler): Observable<number>;
-		timer(dueTime: Date, scheduler: IScheduler): Observable<number>;
+		timer(dueTime: Date, period: number, scheduler?: IScheduler): Observable<number>;
+		timer(dueTime: Date, scheduler?: IScheduler): Observable<number>;
 
 		generateWithAbsoluteTime<TState, TResult>(
 			initialState: TState,