diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -3573,11 +3573,11 @@
     llvm::sys::fs::file_status Status;
     if (llvm::sys::fs::status(A->getValue(), Status))
       D.Diag(diag::err_drv_no_such_file) << A->getValue();
-    CmdArgs.push_back(
-        Args.MakeArgString("-fbuild-session-timestamp=" +
-                           Twine((uint64_t)Status.getLastModificationTime()
-                                     .time_since_epoch()
-                                     .count())));
+    CmdArgs.push_back(Args.MakeArgString(
+        "-fbuild-session-timestamp=" +
+        Twine((uint64_t)std::chrono::duration_cast<std::chrono::seconds>(
+                  Status.getLastModificationTime().time_since_epoch())
+                  .count())));
   }
 
   if (Args.getLastArg(options::OPT_fmodules_validate_once_per_build_session)) {
diff --git a/clang/test/Driver/modules.m b/clang/test/Driver/modules.m
--- a/clang/test/Driver/modules.m
+++ b/clang/test/Driver/modules.m
@@ -11,7 +11,7 @@
 // RUN: %clang -fbuild-session-file=%t.build-session-file -### %s 2>&1 | FileCheck -check-prefix=TIMESTAMP_ONLY %s
 
 // RUN: %clang -fbuild-session-timestamp=1280703457 -### %s 2>&1 | FileCheck -check-prefix=TIMESTAMP_ONLY %s
-// TIMESTAMP_ONLY: -fbuild-session-timestamp=128
+// TIMESTAMP_ONLY: -fbuild-session-timestamp=128{{([[:digit:]]{7})[^[:digit:]]}}
 
 // RUN: %clang -fbuild-session-file=%t.build-session-file -fbuild-session-timestamp=123 -### %s 2>&1 | FileCheck -check-prefix=CONFLICT %s
 // CONFLICT: error: invalid argument '-fbuild-session-file={{.*}}.build-session-file' not allowed with '-fbuild-session-timestamp'
@@ -21,7 +21,7 @@
 // MODULES_VALIDATE_ONCE: -fmodules-validate-once-per-build-session
 
 // RUN: %clang -fbuild-session-file=%t.build-session-file -fmodules-validate-once-per-build-session -### %s 2>&1 | FileCheck -check-prefix=MODULES_VALIDATE_ONCE_FILE %s
-// MODULES_VALIDATE_ONCE_FILE: -fbuild-session-timestamp=128
+// MODULES_VALIDATE_ONCE_FILE: -fbuild-session-timestamp=128{{([[:digit:]]{7})[^[:digit:]]}}
 // MODULES_VALIDATE_ONCE_FILE: -fmodules-validate-once-per-build-session
 
 // RUN: %clang -fmodules-validate-once-per-build-session -### %s 2>&1 | FileCheck -check-prefix=MODULES_VALIDATE_ONCE_ERR %s