tweak

esphome/components/time/posix_tz.cpp

@@ -370,20 +370,22 @@ 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 == ',') {
+    // Second rule is required per POSIX
+    if (*p != ',') {
+      return false;
+    }
     p++;
     if (!internal::parse_dst_rule(p, result.dst_end)) {
       return false;
     }
   }
-  }
 
   return true;
 }

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
 // ============================================================================