diff --git a/ci/test/00_setup_env_native_fuzz_with_valgrind.sh b/ci/test/00_setup_env_native_fuzz_with_valgrind.sh
index 6e1c400d5..45b13a669 100644
--- a/ci/test/00_setup_env_native_fuzz_with_valgrind.sh
+++ b/ci/test/00_setup_env_native_fuzz_with_valgrind.sh
@@ -12,7 +12,7 @@ export NO_DEPENDS=1
 export RUN_UNIT_TESTS=false
 export RUN_FUNCTIONAL_TESTS=false
 export RUN_FUZZ_TESTS=true
-export FUZZ_TESTS_CONFIG="--exclude integer,parse_iso8601 --valgrind"
+export FUZZ_TESTS_CONFIG="--valgrind"
 export GOAL="install"
 export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer CC=clang-8 CXX=clang++-8"
 # Use clang-8, instead of default clang on bionic, which is clang-6 and does not come with libfuzzer on aarch64
diff --git a/src/util/time.cpp b/src/util/time.cpp
index 2afff2626..f33966f14 100644
--- a/src/util/time.cpp
+++ b/src/util/time.cpp
@@ -94,10 +94,12 @@ std::string FormatISO8601DateTime(int64_t nTime) {
     struct tm ts;
     time_t time_val = nTime;
 #ifdef _MSC_VER
-    gmtime_s(&ts, &time_val);
+    if (gmtime_s(&ts, &time_val) != 0) {
 #else
-    gmtime_r(&time_val, &ts);
+    if (gmtime_r(&time_val, &ts) == nullptr) {
 #endif
+        return {};
+    }
     return strprintf("%04i-%02i-%02iT%02i:%02i:%02iZ", ts.tm_year + 1900, ts.tm_mon + 1, ts.tm_mday, ts.tm_hour, ts.tm_min, ts.tm_sec);
 }
 
@@ -105,10 +107,12 @@ std::string FormatISO8601Date(int64_t nTime) {
     struct tm ts;
     time_t time_val = nTime;
 #ifdef _MSC_VER
-    gmtime_s(&ts, &time_val);
+    if (gmtime_s(&ts, &time_val) != 0) {
 #else
-    gmtime_r(&time_val, &ts);
+    if (gmtime_r(&time_val, &ts) == nullptr) {
 #endif
+        return {};
+    }
     return strprintf("%04i-%02i-%02i", ts.tm_year + 1900, ts.tm_mon + 1, ts.tm_mday);
 }
 
@@ -124,4 +128,4 @@ int64_t ParseISO8601DateTime(const std::string& str)
     if (ptime.is_not_a_date_time() || epoch > ptime)
         return 0;
     return (ptime - epoch).total_seconds();
-}
\ No newline at end of file
+}