use int64toa for tv_sec

src/core/security/json_token.c

@@ -215,8 +215,8 @@ static char *encoded_jwt_claim(const grpc_auth_json_key *json_key,
     gpr_log(GPR_INFO, "Cropping token lifetime to maximum allowed value.");
     expiration = gpr_time_add(now, grpc_max_auth_token_lifetime);
   }
-  gpr_ltoa(now.tv_sec, now_str);
-  gpr_ltoa(expiration.tv_sec, expiration_str);
+  gpr_int64toa(now.tv_sec, now_str);
+  gpr_int64toa(expiration.tv_sec, expiration_str);
 
   child =
       create_child(NULL, json, "iss", json_key->client_email, GRPC_JSON_STRING);