diff --git a/src/main.cpp b/src/main.cpp
index 991fa9e..9657050 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -170,9 +170,17 @@ static bool battery_sample_due(uint32_t now_ms) {
 
 static bool sender_timesync_window_due() {
   uint32_t interval_sec = SENDER_TIMESYNC_CHECK_SEC_FAST;
-  if (millis() - g_boot_ms >= 60000UL && time_is_synced() && time_rtc_present()) {
+  bool allow_slow = (millis() - g_boot_ms >= 60000UL) && time_is_synced() && time_rtc_present() &&
+      (g_sender_last_timesync_rx_ms > 0);
+  // RTC boot time is not evidence of receiving network TimeSync.
+  if (allow_slow) {
     interval_sec = SENDER_TIMESYNC_CHECK_SEC_SLOW;
   }
+  static uint32_t last_interval_sec = 0;
+  if (last_interval_sec != interval_sec) {
+    last_interval_sec = interval_sec;
+    g_sender_last_timesync_check_ms = millis();
+  }
   if (g_sender_last_timesync_check_ms == 0) {
     g_sender_last_timesync_check_ms = millis() - interval_sec * 1000UL;
   }