diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -1767,7 +1767,7 @@
 def fbuild_session_timestamp : Joined<["-"], "fbuild-session-timestamp=">,
   Group<i_Group>, Flags<[CC1Option]>, MetaVarName<"<time since Epoch in seconds>">,
   HelpText<"Time when the current build session started">,
-  MarshallingInfoStringInt<"HeaderSearchOpts->BuildSessionTimestamp">;
+  MarshallingInfoStringInt<"HeaderSearchOpts->BuildSessionTimestamp", "0", "uint64_t">;
 def fbuild_session_file : Joined<["-"], "fbuild-session-file=">,
   Group<i_Group>, MetaVarName<"<file>">,
   HelpText<"Use the last modification time of <file> as the build session timestamp">;
diff --git a/clang/unittests/Frontend/CompilerInvocationTest.cpp b/clang/unittests/Frontend/CompilerInvocationTest.cpp
--- a/clang/unittests/Frontend/CompilerInvocationTest.cpp
+++ b/clang/unittests/Frontend/CompilerInvocationTest.cpp
@@ -368,6 +368,18 @@
   ASSERT_THAT(GeneratedArgs, Not(Contains(StrEq("pic"))));
 }
 
+// Wide integer option.
+
+TEST_F(CommandLineTest, WideIntegerHighValue) {
+  const char *Args[] = {"-fbuild-session-timestamp=1609827494445723662"};
+
+  CompilerInvocation::CreateFromArgs(Invocation, Args, *Diags);
+
+  ASSERT_FALSE(Diags->hasErrorOccurred());
+  ASSERT_EQ(Invocation.getHeaderSearchOpts().BuildSessionTimestamp,
+            1609827494445723662ull);
+}
+
 // Tree of boolean options that can be (directly or transitively) implied by
 // their parent:
 //