diff --git a/upb/json_decode.c b/upb/json_decode.c
index 4071fc3d95..556541ccff 100644
--- a/upb/json_decode.c
+++ b/upb/json_decode.c
@@ -631,7 +631,8 @@ static const char* jsondec_buftouint64(jsondec* d, const char* ptr,
 }
 
 static const char* jsondec_buftoint64(jsondec* d, const char* ptr,
-                                      const char* end, int64_t* val) {
+                                      const char* end, int64_t* val,
+                                      bool* is_neg) {
   bool neg = false;
   uint64_t u64;
 
@@ -646,6 +647,9 @@ static const char* jsondec_buftoint64(jsondec* d, const char* ptr,
   }
 
   *val = neg ? -u64 : u64;
+  if (is_neg) {
+    *is_neg = neg;
+  }
   return ptr;
 }
 
@@ -661,7 +665,7 @@ static uint64_t jsondec_strtouint64(jsondec* d, upb_StringView str) {
 static int64_t jsondec_strtoint64(jsondec* d, upb_StringView str) {
   const char* end = str.data + str.size;
   int64_t ret;
-  if (jsondec_buftoint64(d, str.data, end, &ret) != end) {
+  if (jsondec_buftoint64(d, str.data, end, &ret, NULL) != end) {
     jsondec_err(d, "Non-number characters in quoted integer");
   }
   return ret;
@@ -1130,9 +1134,10 @@ static void jsondec_duration(jsondec* d, upb_Message* msg,
   const char* ptr = str.data;
   const char* end = ptr + str.size;
   const int64_t max = (uint64_t)3652500 * 86400;
+  bool neg = false;
 
   /* "3.000000001s", "3s", etc. */
-  ptr = jsondec_buftoint64(d, ptr, end, &seconds.int64_val);
+  ptr = jsondec_buftoint64(d, ptr, end, &seconds.int64_val, &neg);
   nanos.int32_val = jsondec_nanos(d, &ptr, end);
 
   if (end - ptr != 1 || *ptr != 's') {
@@ -1143,7 +1148,7 @@ static void jsondec_duration(jsondec* d, upb_Message* msg,
     jsondec_err(d, "Duration out of range");
   }
 
-  if (seconds.int64_val < 0) {
+  if (neg) {
     nanos.int32_val = -nanos.int32_val;
   }
 
diff --git a/upb/json_encode.c b/upb/json_encode.c
index 7836b47b1b..468b7f897c 100644
--- a/upb/json_encode.c
+++ b/upb/json_encode.c
@@ -188,17 +188,27 @@ static void jsonenc_duration(jsonenc* e, const upb_Message* msg,
   const upb_FieldDef* nanos_f = upb_MessageDef_FindFieldByNumber(m, 2);
   int64_t seconds = upb_Message_Get(msg, seconds_f).int64_val;
   int32_t nanos = upb_Message_Get(msg, nanos_f).int32_val;
+  bool negative = false;
 
   if (seconds > 315576000000 || seconds < -315576000000 ||
-      (seconds < 0) != (nanos < 0)) {
+      (seconds != 0 && nanos != 0 && (seconds < 0) != (nanos < 0))) {
     jsonenc_err(e, "bad duration");
   }
 
+  if (seconds < 0) {
+    negative = true;
+    seconds = -seconds;
+  }
   if (nanos < 0) {
+    negative = true;
     nanos = -nanos;
   }
 
-  jsonenc_printf(e, "\"%" PRId64, seconds);
+  jsonenc_putstr(e, "\"");
+  if (negative) {
+    jsonenc_putstr(e, "-");
+  }
+  jsonenc_printf(e, "%" PRId64, seconds);
   jsonenc_nanos(e, nanos);
   jsonenc_putstr(e, "s\"");
 }