diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 857ac30656a..eb1a515f24d 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -63,7 +63,7 @@ namespace ts {
     interface OutputFingerprint {
         hash: string;
         byteOrderMark: boolean;
-        mtime: Date;
+        mtime: Date | undefined;
     }
 
     export function createCompilerHost(options: CompilerOptions, setParentNodes?: boolean): CompilerHost {
@@ -128,6 +128,7 @@ namespace ts {
                 if (fingerprint &&
                     fingerprint.byteOrderMark === writeByteOrderMark &&
                     fingerprint.hash === hash &&
+                    fingerprint.mtime !== undefined &&
                     fingerprint.mtime.getTime() === mtimeBefore.getTime()) {
                     return;
                 }
diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index 4edba26333a..8f6a58a3567 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -482,7 +482,7 @@ namespace ts {
         getCurrentDirectory(): string;
         getDirectories(path: string): string[];
         readDirectory(path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): string[];
-        getModifiedTime?(path: string): Date;
+        getModifiedTime?(path: string): Date | undefined;
         setModifiedTime?(path: string, time: Date): void;
         deleteFile?(path: string): void;
         /**
diff --git a/src/compiler/tsbuild.ts b/src/compiler/tsbuild.ts
index 5c9c7086db8..d98a161113b 100644
--- a/src/compiler/tsbuild.ts
+++ b/src/compiler/tsbuild.ts
@@ -848,9 +848,14 @@ namespace ts {
             const outputs = getAllProjectOutputs(proj);
             let priorNewestUpdateTime = minimumDate;
             for (const file of outputs) {
+
                 if (isDeclarationFile(file)) {
-                    priorNewestUpdateTime = newer(priorNewestUpdateTime, compilerHost.getModifiedTime!(file));
+                    const fileModifiedTime = compilerHost.getModifiedTime!(file);
+                    if (fileModifiedTime !== undefined) {
+                        priorNewestUpdateTime = newer(priorNewestUpdateTime, fileModifiedTime);
+                    }
                 }
+
                 compilerHost.setModifiedTime!(file, now);
             }
 
@@ -1058,7 +1063,7 @@ namespace ts {
             }
 
             const inputTime = host.getModifiedTime(inputFile);
-            if (inputTime > newestInputFileTime) {
+            if (inputTime !== undefined && inputTime > newestInputFileTime) {
                 newestInputFileName = inputFile;
                 newestInputFileTime = inputTime;
             }
@@ -1090,19 +1095,19 @@ namespace ts {
             }
 
             const outputTime = host.getModifiedTime(output);
-            if (outputTime < oldestOutputFileTime) {
+            if (outputTime !== undefined && outputTime < oldestOutputFileTime) {
                 oldestOutputFileTime = outputTime;
                 oldestOutputFileName = output;
             }
 
             // If an output is older than the newest input, we can stop checking
             // Don't immediately return because we can still be upstream-blocked, which is a higher-priority status
-            if (outputTime < newestInputFileTime) {
+            if (outputTime !== undefined && outputTime < newestInputFileTime) {
                 isOutOfDateWithInputs = true;
                 break;
             }
 
-            if (outputTime > newestOutputFileTime) {
+            if (outputTime !== undefined && outputTime > newestOutputFileTime) {
                 newestOutputFileTime = outputTime;
                 newestOutputFileName = output;
             }
@@ -1117,7 +1122,10 @@ namespace ts {
                     newestDeclarationFileContentChangedTime = newer(unchangedTime, newestDeclarationFileContentChangedTime);
                 }
                 else {
-                    newestDeclarationFileContentChangedTime = newer(newestDeclarationFileContentChangedTime, host.getModifiedTime(output));
+                    const outputModifiedTime = host.getModifiedTime(output);
+                    if (outputModifiedTime !== undefined) {
+                        newestDeclarationFileContentChangedTime = newer(newestDeclarationFileContentChangedTime, outputModifiedTime);
+                    }
                 }
             }
         }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index eb8a0c061d5..d0a5add280e 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4682,7 +4682,7 @@ namespace ts {
 
     export interface UpToDateHost {
         fileExists(fileName: string): boolean;
-        getModifiedTime(fileName: string): Date;
+        getModifiedTime(fileName: string): Date | undefined;
         getUnchangedTime?(fileName: string): Date | undefined;
         getLastStatus?(fileName: string): UpToDateStatus | undefined;
         setLastStatus?(fileName: string, status: UpToDateStatus): void;
@@ -4820,7 +4820,7 @@ namespace ts {
         /* @internal */ hasChangedAutomaticTypeDirectiveNames?: boolean;
         createHash?(data: string): string;
 
-        getModifiedTime?(fileName: string): Date;
+        getModifiedTime?(fileName: string): Date | undefined;
         setModifiedTime?(fileName: string, date: Date): void;
         deleteFile?(fileName: string): void;
     }
diff --git a/src/tsserver/server.ts b/src/tsserver/server.ts
index 17f38c3c7ce..741d3c2f8a1 100644
--- a/src/tsserver/server.ts
+++ b/src/tsserver/server.ts
@@ -676,7 +676,7 @@ namespace ts.server {
             fs.stat(watchedFile.fileName, (err, stats) => {
                 if (err) {
                     if (err.code === "ENOENT") {
-                        if (watchedFile.mtime.getTime() !== 0) {
+                        if (watchedFile.mtime !== undefined && watchedFile.mtime.getTime() !== 0) {
                             watchedFile.mtime = missingFileModifiedTime;
                             watchedFile.callback(watchedFile.fileName, FileWatcherEventKind.Deleted);
                         }