diff --git a/src/main.c b/src/main.c
--- a/src/main.c
+++ b/src/main.c
@@ -64,7 +64,7 @@ int main(void)
     uint32_t fix_acq_starttime = 0;
     uint32_t nextwspr_time = 0;
     uint8_t nextwspr_time_valid = 0;
-    uint32_t last_wspr_tx_time = 0;
+    uint64_t last_wspr_tx_time = 0;
     uint64_t idle_blink_last = 0;
 
     uint8_t packet_type = 0;
@@ -82,7 +82,7 @@ int main(void)
     while (1)
     {
     	// Every 10 minutes, wake up and try to wspr
-    	if(state == SYSTEM_IDLE && (HAL_GetTick() - last_wspr_tx_time > 60000 * 10))
+    	if(state == SYSTEM_IDLE && (rtc_timestamp() - last_wspr_tx_time > 60000 * 10))
     	{
     		state = SYSTEM_GPSACQ;
     	}
@@ -182,7 +182,7 @@ int main(void)
                 	led_blink(4);
                     gps_poweroff();
                     fix_acq_starttime = 0;
-                    last_wspr_tx_time = HAL_GetTick(); // repeat acq/tx cycle after big time delay
+                    last_wspr_tx_time = rtc_timestamp(); // repeat acq/tx cycle after big time delay
                 	state = SYSTEM_IDLE;
                 }
                 else
@@ -216,14 +216,14 @@ int main(void)
                         // TODO: Switch between alternate and standard packet
 						wspr_transmit(grid_locator, packet_type);
                         packet_type = !packet_type; // alternate packet type
-						last_wspr_tx_time = HAL_GetTick();
+						last_wspr_tx_time = rtc_timestamp();
 						state = SYSTEM_IDLE;
                         adc_stop();
             		}
             		else
             		{
             			// Window was missed, go back to idle, and try again after time delay
-						last_wspr_tx_time = HAL_GetTick();
+						last_wspr_tx_time = rtc_timestamp();
             			state = SYSTEM_IDLE;
                         adc_stop();
             		}