From bec7d6d223c48be4e35e5ac90eb55fbf7fe09c12 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Thu, 29 Jan 2026 22:31:23 -0600
Subject: [PATCH] tweak

---
 esphome/components/time/posix_tz.cpp      | 14 ++++++++------
 tests/components/time/posix_tz_parser.cpp | 18 ++++++++++++++++++
 2 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/esphome/components/time/posix_tz.cpp b/esphome/components/time/posix_tz.cpp
index 7a122b474a..f91c168811 100644
--- a/esphome/components/time/posix_tz.cpp
+++ b/esphome/components/time/posix_tz.cpp
@@ -370,18 +370,20 @@ bool parse_posix_tz(const char *tz_string, ParsedTimezone &result) {
     result.dst_offset_seconds = result.std_offset_seconds - 3600;
   }
 
-  // Parse DST rules if present
+  // Parse DST rules if present (POSIX requires both start and end if any rules specified)
   if (*p == ',') {
     p++;
     if (!internal::parse_dst_rule(p, result.dst_start)) {
       return false;
     }
 
-    if (*p == ',') {
-      p++;
-      if (!internal::parse_dst_rule(p, result.dst_end)) {
-        return false;
-      }
+    // Second rule is required per POSIX
+    if (*p != ',') {
+      return false;
+    }
+    p++;
+    if (!internal::parse_dst_rule(p, result.dst_end)) {
+      return false;
     }
   }
 
diff --git a/tests/components/time/posix_tz_parser.cpp b/tests/components/time/posix_tz_parser.cpp
index 38f8640ec9..7badaa37f0 100644
--- a/tests/components/time/posix_tz_parser.cpp
+++ b/tests/components/time/posix_tz_parser.cpp
@@ -361,6 +361,24 @@ TEST(PosixTzParser, MFormatInvalidDayOfWeek7) {
   EXPECT_FALSE(parse_posix_tz("EST5EDT,M3.2.7,M11.1.0", tz));
 }
 
+TEST(PosixTzParser, MissingEndRule) {
+  ParsedTimezone tz;
+  // POSIX requires both start and end rules if any rules are specified
+  EXPECT_FALSE(parse_posix_tz("EST5EDT,M3.2.0", tz));
+}
+
+TEST(PosixTzParser, MissingEndRuleJFormat) {
+  ParsedTimezone tz;
+  // POSIX requires both start and end rules if any rules are specified
+  EXPECT_FALSE(parse_posix_tz("EST5EDT,J60", tz));
+}
+
+TEST(PosixTzParser, MissingEndRulePlainDay) {
+  ParsedTimezone tz;
+  // POSIX requires both start and end rules if any rules are specified
+  EXPECT_FALSE(parse_posix_tz("EST5EDT,60", tz));
+}
+
 // ============================================================================
 // Large offset tests
 // ============================================================================