diff --git a/preprocess.c b/preprocess.c
index 715830d..7b40774 100644
--- a/preprocess.c
+++ b/preprocess.c
@@ -893,6 +893,20 @@ static Token *counter_macro(Token *tmpl) {
   return new_num_token(i++, tmpl);
 }
 
+// __TIMESTAMP__ is expanded to a string describing the last
+// modification time of the current file. E.g.
+// "Fri Jul 24 01:32:50 2020"
+static Token *timestamp_macro(Token *tmpl) {
+  struct stat st;
+  if (stat(tmpl->file->name, &st) != 0)
+    return new_str_token("??? ??? ?? ??:??:?? ????", tmpl);
+
+  char buf[30];
+  ctime_r(&st.st_mtime, buf);
+  buf[24] = '\0';
+  return new_str_token(buf, tmpl);
+}
+
 // __DATE__ is expanded to the current date, e.g. "May 17 2020".
 static char *format_date(struct tm *tm) {
   static char mon[][4] = {
@@ -957,6 +971,7 @@ void init_macros(void) {
   add_builtin("__FILE__", file_macro);
   add_builtin("__LINE__", line_macro);
   add_builtin("__COUNTER__", counter_macro);
+  add_builtin("__TIMESTAMP__", timestamp_macro);
 
   time_t now = time(NULL);
   struct tm *tm = localtime(&now);
diff --git a/test/macro.c b/test/macro.c
index 700865d..1f8f13d 100644
--- a/test/macro.c
+++ b/test/macro.c
@@ -373,6 +373,8 @@ int main() {
   ASSERT(1, __COUNTER__);
   ASSERT(2, __COUNTER__);
 
+  ASSERT(24, strlen(__TIMESTAMP__));
+
   printf("OK\n");
   return 0;
 }