From f286bc57f3fda499f270c37e4390c2189a3980ec Mon Sep 17 00:00:00 2001
From: Eyal <109809+eyal0@users.noreply.github.com>
Date: Mon, 1 Sep 2025 22:45:25 -0600
Subject: [PATCH] [core] Fix timezone offset calculation (#10426)

---
 esphome/core/time.cpp | 26 ++++++--------------------
 1 file changed, 6 insertions(+), 20 deletions(-)

diff --git a/esphome/core/time.cpp b/esphome/core/time.cpp
index f9652b5329..fe6f50158c 100644
--- a/esphome/core/time.cpp
+++ b/esphome/core/time.cpp
@@ -203,27 +203,13 @@ void ESPTime::recalc_timestamp_local() {
 }
 
 int32_t ESPTime::timezone_offset() {
-  int32_t offset = 0;
   time_t now = ::time(nullptr);
-  auto local = ESPTime::from_epoch_local(now);
-  auto utc = ESPTime::from_epoch_utc(now);
-  bool negative = utc.hour > local.hour && local.day_of_year <= utc.day_of_year;
-
-  if (utc.minute > local.minute) {
-    local.minute += 60;
-    local.hour -= 1;
-  }
-  offset += (local.minute - utc.minute) * 60;
-
-  if (negative) {
-    offset -= (utc.hour - local.hour) * 3600;
-  } else {
-    if (utc.hour > local.hour) {
-      local.hour += 24;
-    }
-    offset += (local.hour - utc.hour) * 3600;
-  }
-  return offset;
+  struct tm local_tm = *::localtime(&now);
+  local_tm.tm_isdst = 0;  // Cause mktime to ignore daylight saving time because we want to include it in the offset.
+  time_t local_time = mktime(&local_tm);
+  struct tm utc_tm = *::gmtime(&now);
+  time_t utc_time = mktime(&utc_tm);
+  return static_cast<int32_t>(local_time - utc_time);
 }
 
 bool ESPTime::operator<(const ESPTime &other) const { return this->timestamp < other.timestamp; }