diff --git a/esphome/components/shelly_dimmer/stm32flash.cpp b/esphome/components/shelly_dimmer/stm32flash.cpp
index b052c0cee9..a1a933bcab 100644
--- a/esphome/components/shelly_dimmer/stm32flash.cpp
+++ b/esphome/components/shelly_dimmer/stm32flash.cpp
@@ -149,7 +149,7 @@ stm32_err_t stm32_get_ack_timeout(const stm32_unique_ptr &stm, uint32_t timeout)
   do {
     yield();
     if (!stream->available()) {
-      if (millis() < start_time + timeout)
+      if (millis() - start_time < timeout)
         continue;
       ESP_LOGD(TAG, "Failed to read ACK timeout=%i", timeout);
       return STM32_ERR_UNKNOWN;
@@ -212,7 +212,7 @@ stm32_err_t stm32_resync(const stm32_unique_ptr &stm) {
   static_assert(sizeof(buf) == BUFFER_SIZE, "Buf expected to be 2 bytes");
 
   uint8_t ack;
-  while (t1 < t0 + STM32_RESYNC_TIMEOUT) {
+  while (t1 - t0 < STM32_RESYNC_TIMEOUT) {
     stream->write_array(buf, BUFFER_SIZE);
     stream->flush();
     if (!stream->read_array(&ack, 1)) {