diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -935,6 +935,14 @@
                   ArgSpec<CharPtr>,
               ]
           >,
+          FunctionSpec<
+              "difftime",
+              RetValSpec<DoubleType>,
+              [
+                  ArgSpec<TimeTType>,
+                  ArgSpec<TimeTType>,
+              ]
+          >,
           FunctionSpec<
               "gmtime",
               RetValSpec<StructTmPtr>,