diff --git a/src/shrpx.cc b/src/shrpx.cc
index a36c168b..c252f61f 100644
--- a/src/shrpx.cc
+++ b/src/shrpx.cc
@@ -1959,7 +1959,8 @@ void fill_default_config(Config *config) {
       abort();
     }
 
-    upstreamconf.initial_rtt = NGTCP2_DEFAULT_INITIAL_RTT;
+    upstreamconf.initial_rtt =
+        static_cast<ev_tstamp>(NGTCP2_DEFAULT_INITIAL_RTT) / NGTCP2_SECONDS;
   }
 
   auto &http3conf = config->http3;
@@ -3404,10 +3405,7 @@ HTTP/3 and QUIC:
   --frontend-quic-initial-rtt=<DURATION>
               Specify the initial RTT of the frontend QUIC connection.
               Default: )"
-      << util::duration_str(
-             static_cast<double>(config->quic.upstream.initial_rtt) /
-             NGTCP2_SECONDS)
-      << R"(
+      << util::duration_str(config->quic.upstream.initial_rtt) << R"(
   --no-quic-bpf
               Disable eBPF.
   --frontend-http3-window-size=<SIZE>
diff --git a/src/shrpx_config.cc b/src/shrpx_config.cc
index 25576c70..90bdc1c2 100644
--- a/src/shrpx_config.cc
+++ b/src/shrpx_config.cc
@@ -4159,13 +4159,7 @@ int parse_config(Config *config, int optid, const StringRef &opt,
                           optarg);
   case SHRPX_OPTID_FRONTEND_QUIC_INITIAL_RTT: {
 #ifdef ENABLE_HTTP3
-    ev_tstamp d;
-    if (parse_duration(&d, opt, optarg) != 0) {
-      return -1;
-    }
-
-    config->quic.upstream.initial_rtt =
-        static_cast<ngtcp2_duration>(d * NGTCP2_SECONDS);
+    return parse_duration(&config->quic.upstream.initial_rtt, opt, optarg);
 #endif // ENABLE_HTTP3
 
     return 0;
diff --git a/src/shrpx_config.h b/src/shrpx_config.h
index 1b19d2f5..27b6a9ac 100644
--- a/src/shrpx_config.h
+++ b/src/shrpx_config.h
@@ -782,7 +782,7 @@ struct QUICConfig {
     bool require_token;
     std::array<uint8_t, SHRPX_QUIC_SERVER_IDLEN> server_id;
     StringRef secret_file;
-    ngtcp2_duration initial_rtt;
+    ev_tstamp initial_rtt;
   } upstream;
   struct {
     StringRef prog_file;
diff --git a/src/shrpx_http3_upstream.cc b/src/shrpx_http3_upstream.cc
index f59d0fff..662c19ae 100644
--- a/src/shrpx_http3_upstream.cc
+++ b/src/shrpx_http3_upstream.cc
@@ -592,7 +592,8 @@ int Http3Upstream::init(const UpstreamAddr *faddr, const Address &remote_addr,
   }
 
   settings.initial_ts = quic_timestamp();
-  settings.initial_rtt = quicconf.upstream.initial_rtt;
+  settings.initial_rtt = static_cast<ngtcp2_tstamp>(
+      quicconf.upstream.initial_rtt * NGTCP2_SECONDS);
   settings.cc_algo = quicconf.upstream.congestion_controller;
   settings.max_window = http3conf.upstream.max_connection_window_size;
   settings.max_stream_window = http3conf.upstream.max_window_size;