diff --git a/src/library.js b/src/library.js
index a192a7b184a16..64a46577c5e41 100644
--- a/src/library.js
+++ b/src/library.js
@@ -1024,11 +1024,16 @@ LibraryManager.library = {
         return '%';
       }
     };
+
+    // Replace %% with a pair of NULLs (which cannot occur in a C string), then
+    // re-inject them after processing.
+    pattern = pattern.replace(/%%/g, '\0\0')
     for (var rule in EXPANSION_RULES_2) {
       if (pattern.includes(rule)) {
         pattern = pattern.replace(new RegExp(rule, 'g'), EXPANSION_RULES_2[rule](date));
       }
     }
+    pattern = pattern.replace(/\0\0/g, '%')
 
     var bytes = intArrayFromString(pattern, false);
     if (bytes.length > maxsize) {
diff --git a/tests/core/test_strftime.cpp b/tests/core/test_strftime.cpp
index 4ba7c575fc929..c99603a0ec8e8 100644
--- a/tests/core/test_strftime.cpp
+++ b/tests/core/test_strftime.cpp
@@ -39,6 +39,11 @@ int main() {
   tm.tm_yday = 51;
   tm.tm_isdst = 0;
 
+  // Test %% escaping
+  const char *fmt = "%%H=%H %%M=%m %%z=%z";
+  strftime(s, sizeof(s), fmt, &tm);
+  printf("%s -> %s\n", fmt, s);
+
   size = strftime(s, 1000, "", &tm);
   test((size == 0) && (*s == '\0'), "strftime test #1", s);
 
diff --git a/tests/core/test_strftime.out b/tests/core/test_strftime.out
index 86f916ae39a3a..d25f92e6c8448 100644
--- a/tests/core/test_strftime.out
+++ b/tests/core/test_strftime.out
@@ -1,3 +1,4 @@
+%%H=%H %%M=%m %%z=%z -> %H=20 %M=02 %z=+0000
 strftime test #1: 1
 strftime test #2: 1
 strftime test #3: 1