diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 334c054a8fb..7b31d09ad68 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -135,7 +135,7 @@ namespace ts {
 
             sys.writeFile(fileName, data, writeByteOrderMark);
 
-            const mtimeAfter = sys.getModifiedTime!(fileName); // TODO: GH#18217
+            const mtimeAfter = sys.getModifiedTime!(fileName) || missingFileModifiedTime; // TODO: GH#18217
 
             outputFingerprints.set(fileName, {
                 hash,
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..205e7cd8940 100644
--- a/src/compiler/tsbuild.ts
+++ b/src/compiler/tsbuild.ts
@@ -843,14 +843,18 @@ namespace ts {
                 return buildHost.message(Diagnostics.A_non_dry_build_would_build_project_0, proj.options.configFilePath!);
             }
 
-            if (context.options.verbose) buildHost.verbose(Diagnostics.Updating_output_timestamps_of_project_0, proj.options.configFilePath!);
+            if (context.options.verbose) {
+                buildHost.verbose(Diagnostics.Updating_output_timestamps_of_project_0, proj.options.configFilePath!);
+            }
+
             const now = new Date();
             const outputs = getAllProjectOutputs(proj);
             let priorNewestUpdateTime = minimumDate;
             for (const file of outputs) {
                 if (isDeclarationFile(file)) {
-                    priorNewestUpdateTime = newer(priorNewestUpdateTime, compilerHost.getModifiedTime!(file));
+                    priorNewestUpdateTime = newer(priorNewestUpdateTime, compilerHost.getModifiedTime!(file) || missingFileModifiedTime);
                 }
+
                 compilerHost.setModifiedTime!(file, now);
             }
 
@@ -1057,7 +1061,7 @@ namespace ts {
                 };
             }
 
-            const inputTime = host.getModifiedTime(inputFile);
+            const inputTime = host.getModifiedTime(inputFile) || missingFileModifiedTime;
             if (inputTime > newestInputFileTime) {
                 newestInputFileName = inputFile;
                 newestInputFileTime = inputTime;
@@ -1089,7 +1093,7 @@ namespace ts {
                 break;
             }
 
-            const outputTime = host.getModifiedTime(output);
+            const outputTime = host.getModifiedTime(output) || missingFileModifiedTime;
             if (outputTime < oldestOutputFileTime) {
                 oldestOutputFileTime = outputTime;
                 oldestOutputFileName = output;
@@ -1117,7 +1121,8 @@ namespace ts {
                     newestDeclarationFileContentChangedTime = newer(unchangedTime, newestDeclarationFileContentChangedTime);
                 }
                 else {
-                    newestDeclarationFileContentChangedTime = newer(newestDeclarationFileContentChangedTime, host.getModifiedTime(output));
+                    const outputModifiedTime = host.getModifiedTime(output) || missingFileModifiedTime;
+                    newestDeclarationFileContentChangedTime = newer(newestDeclarationFileContentChangedTime, outputModifiedTime);
                 }
             }
         }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 54f85349bf1..87664652b35 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4684,7 +4684,7 @@ 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;
@@ -4822,7 +4822,7 @@ 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/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index eede4badc7e..5118830e840 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -2584,7 +2584,7 @@ declare namespace ts {
     }
     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;
@@ -2688,7 +2688,7 @@ declare namespace ts {
         resolveTypeReferenceDirectives?(typeReferenceDirectiveNames: string[], containingFile: string): ResolvedTypeReferenceDirective[];
         getEnvironmentVariable?(name: string): string | undefined;
         createHash?(data: string): string;
-        getModifiedTime?(fileName: string): Date;
+        getModifiedTime?(fileName: string): Date | undefined;
         setModifiedTime?(fileName: string, date: Date): void;
         deleteFile?(fileName: string): void;
     }
@@ -3025,7 +3025,7 @@ declare 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/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index bf75c37782b..488f7c85a69 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -2584,7 +2584,7 @@ declare namespace ts {
     }
     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;
@@ -2688,7 +2688,7 @@ declare namespace ts {
         resolveTypeReferenceDirectives?(typeReferenceDirectiveNames: string[], containingFile: string): ResolvedTypeReferenceDirective[];
         getEnvironmentVariable?(name: string): string | undefined;
         createHash?(data: string): string;
-        getModifiedTime?(fileName: string): Date;
+        getModifiedTime?(fileName: string): Date | undefined;
         setModifiedTime?(fileName: string, date: Date): void;
         deleteFile?(fileName: string): void;
     }
@@ -3025,7 +3025,7 @@ declare 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;
         /**