diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index 8aaff65febc..96f516632c7 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -641,6 +641,14 @@ namespace Harness.LanguageService {
 
         startGroup(): void {
         }
+
+        setTimeout(callback: (...args: any[]) => void, ms: number, ...args: any[]): any {
+            return setTimeout(callback, ms, args);
+        }
+
+        clearTimeout(timeoutId: any): void {
+            clearTimeout(timeoutId);
+        }
     }
 
     export class ServerLanguageServiceAdapter implements LanguageServiceAdapter {
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index 77908779d9f..793e17d52e7 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -533,7 +533,7 @@ namespace ts.server {
         // number becomes 0 for a watcher, then we should close it.
         directoryWatchersRefCount: ts.Map<number> = {};
         hostConfiguration: HostConfiguration;
-        timerForDetectingProjectFileListChanges: Map<NodeJS.Timer> = {};
+        timerForDetectingProjectFileListChanges: Map<any> = {};
 
         constructor(public host: ServerHost, public psLogger: Logger, public eventHandler?: ProjectServiceEventHandler) {
             // ts.disableIncrementalParsing = true;
@@ -593,9 +593,9 @@ namespace ts.server {
 
         startTimerForDetectingProjectFileListChanges(project: Project) {
             if (this.timerForDetectingProjectFileListChanges[project.projectFilename]) {
-                clearTimeout(this.timerForDetectingProjectFileListChanges[project.projectFilename]);
+                this.host.clearTimeout(this.timerForDetectingProjectFileListChanges[project.projectFilename]);
             }
-            this.timerForDetectingProjectFileListChanges[project.projectFilename] = setTimeout(
+            this.timerForDetectingProjectFileListChanges[project.projectFilename] = this.host.setTimeout(
                 () => this.handleProjectFileListChanges(project),
                 250
             );
diff --git a/src/server/server.ts b/src/server/server.ts
index ec83f8cc716..767793024c2 100644
--- a/src/server/server.ts
+++ b/src/server/server.ts
@@ -266,16 +266,21 @@ namespace ts.server {
         }
     }
 
+    const sys = <ServerHost>ts.sys;
+
     // Override sys.write because fs.writeSync is not reliable on Node 4
-    ts.sys.write = (s: string) => writeMessage(s);
-    ts.sys.watchFile = (fileName, callback) => {
+    sys.write = (s: string) => writeMessage(s);
+    sys.watchFile = (fileName, callback) => {
         const watchedFile = pollingWatchedFileSet.addFile(fileName, callback);
         return {
             close: () => pollingWatchedFileSet.removeFile(watchedFile)
         };
     };
 
-    const ioSession = new IOSession(ts.sys, logger);
+    sys.setTimeout = setTimeout;
+    sys.clearTimeout = clearTimeout;
+
+    const ioSession = new IOSession(sys, logger);
     process.on("uncaughtException", function(err: Error) {
         ioSession.logError(err, "unknown");
     });
diff --git a/src/server/session.ts b/src/server/session.ts
index 8c6525a70e6..09b86da48cc 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -133,6 +133,8 @@ namespace ts.server {
     }
 
     export interface ServerHost extends ts.System {
+        setTimeout(callback: (...args: any[]) => void, ms: number, ...args: any[]): any;
+        clearTimeout(timeoutId: any): void;
     }
 
     export class Session {
diff --git a/tests/cases/unittests/cachingInServerLSHost.ts b/tests/cases/unittests/cachingInServerLSHost.ts
index 58bcd76e246..0d14113942c 100644
--- a/tests/cases/unittests/cachingInServerLSHost.ts
+++ b/tests/cases/unittests/cachingInServerLSHost.ts
@@ -60,6 +60,12 @@ module ts {
                 return {
                     close: () => { }
                 }
+            },
+            setTimeout: (callback, ms, ...args) => {
+                return setTimeout(callback, ms, args);
+            },
+            clearTimeout: (timeoutId) => {
+                clearTimeout(timeoutId);
             }
         };
     }
diff --git a/tests/cases/unittests/session.ts b/tests/cases/unittests/session.ts
index ee9742482cd..a90f7d2ec84 100644
--- a/tests/cases/unittests/session.ts
+++ b/tests/cases/unittests/session.ts
@@ -18,7 +18,9 @@ namespace ts.server {
         getExecutingFilePath(): string { return void 0; },
         getCurrentDirectory(): string { return void 0; },
         readDirectory(): string[] { return []; },
-        exit(): void {}
+        exit(): void { },
+        setTimeout(callback, ms, ...args) { return 0; },
+        clearTimeout(timeoutId) { }
     };
     const mockLogger: Logger = {
         close(): void {},
diff --git a/tests/cases/unittests/versionCache.ts b/tests/cases/unittests/versionCache.ts
index c1d283ca8cd..4c2b21224c8 100644
--- a/tests/cases/unittests/versionCache.ts
+++ b/tests/cases/unittests/versionCache.ts
@@ -271,7 +271,7 @@ and grew 1cm per day`;
         });
 
         it("Edit ScriptVersionCache ", () => {
-            let svc = server.ScriptVersionCache.fromString(ts.sys, testContent);
+            let svc = server.ScriptVersionCache.fromString(<server.ServerHost>ts.sys, testContent);
             let checkText = testContent;
 
             for (let i = 0; i < iterationCount; i++) {