[datetime] Fix state_as_esptime() returning invalid timestamp (#15128)

esphome/components/datetime/datetime_entity.cpp

@@ -60,6 +60,9 @@ ESPTime DateTimeEntity::state_as_esptime() const {
   obj.year = this->year_;
   obj.month = this->month_;
   obj.day_of_month = this->day_;
+  obj.day_of_week = 0;
+  obj.day_of_year = 0;
+  obj.is_dst = false;
   obj.hour = this->hour_;
   obj.minute = this->minute_;
   obj.second = this->second_;

esphome/core/time.cpp

@@ -231,7 +231,7 @@ void ESPTime::increment_day() {
 
 void ESPTime::recalc_timestamp_utc(bool use_day_of_year) {
   time_t res = 0;
-  if (!this->fields_in_range()) {
+  if (!this->fields_in_range(false, use_day_of_year)) {
     this->timestamp = -1;
     return;
   }

esphome/core/time.h

@@ -79,11 +79,19 @@ struct ESPTime {
   /// Check if this ESPTime is valid (all fields in range and year is greater than or equal to 2019)
   bool is_valid() const { return this->year >= 2019 && this->fields_in_range(); }
 
-  /// Check if all time fields of this ESPTime are in range.
-  bool fields_in_range() const {
-    return this->second < 61 && this->minute < 60 && this->hour < 24 && this->day_of_week > 0 &&
-           this->day_of_week < 8 && this->day_of_year > 0 && this->day_of_year < 367 && this->month > 0 &&
-           this->month < 13 && this->day_of_month > 0 && this->day_of_month <= days_in_month(this->month, this->year);
+  /// Check if time fields are in range.
+  /// @param check_day_of_week validate day_of_week (not always available when constructing from date/time fields)
+  /// @param check_day_of_year validate day_of_year (not always available when constructing from date/time fields)
+  bool fields_in_range(bool check_day_of_week = true, bool check_day_of_year = true) const {
+    bool valid = this->second < 61 && this->minute < 60 && this->hour < 24 && this->month > 0 && this->month < 13 &&
+                 this->day_of_month > 0 && this->day_of_month <= days_in_month(this->month, this->year);
+    if (check_day_of_week) {
+      valid = valid && this->day_of_week > 0 && this->day_of_week < 8;
+    }
+    if (check_day_of_year) {
+      valid = valid && this->day_of_year > 0 && this->day_of_year < 367;
+    }
+    return valid;
   }
 
   /** Convert a string to ESPTime struct as specified by the format argument.

tests/components/time/posix_tz_parser.cpp

@@ -1036,8 +1036,6 @@ static time_t esptime_recalc_local(int year, int month, int day, int hour, int m
   t.hour = hour;
   t.minute = min;
   t.second = sec;
-  t.day_of_week = 1;  // Placeholder for fields_in_range()
-  t.day_of_year = 1;
   t.recalc_timestamp_local();
   return t.timestamp;
 }
@@ -1187,6 +1185,60 @@ TEST(RecalcTimestampLocal, NonDefaultTransitionTime) {
   EXPECT_EQ(esp_result, libc_result);
 }
 
+TEST(RecalcTimestampLocal, MinimalFieldsWithoutDayOfWeekOrYear) {
+  // Regression test for issue #15115: DateTimeEntity::state_as_esptime() constructs
+  // an ESPTime with only year/month/day/hour/minute/second set (no day_of_week or
+  // day_of_year). recalc_timestamp_local() must work without those fields.
+  const char *tz_str = "CET-1CEST,M3.5.0,M10.5.0";
+  setenv("TZ", tz_str, 1);
+  tzset();
+  time::ParsedTimezone tz{};
+  ASSERT_TRUE(parse_posix_tz(tz_str, tz));
+  set_global_tz(tz);
+
+  // Construct ESPTime with only date/time fields (like state_as_esptime does)
+  ESPTime t{};
+  t.year = 2026;
+  t.month = 3;
+  t.day_of_month = 20;
+  t.hour = 23;
+  t.minute = 14;
+  t.second = 55;
+  // day_of_week and day_of_year are deliberately left as 0
+  t.recalc_timestamp_local();
+
+  // Must NOT return -1 (the bug: fields_in_range() rejected valid times)
+  EXPECT_NE(t.timestamp, -1);
+
+  // Verify against libc
+  time_t libc_result = libc_mktime(2026, 3, 20, 23, 14, 55);
+  EXPECT_EQ(t.timestamp, libc_result);
+}
+
+TEST(RecalcTimestampLocal, MinimalFieldsNoDST) {
+  // Same test but with a timezone that has no DST
+  const char *tz_str = "IST-5:30";
+  setenv("TZ", tz_str, 1);
+  tzset();
+  time::ParsedTimezone tz{};
+  ASSERT_TRUE(parse_posix_tz(tz_str, tz));
+  set_global_tz(tz);
+
+  ESPTime t{};
+  t.year = 2026;
+  t.month = 3;
+  t.day_of_month = 23;
+  t.hour = 10;
+  t.minute = 0;
+  t.second = 0;
+  t.recalc_timestamp_local();
+
+  EXPECT_NE(t.timestamp, -1);
+
+  time_t libc_result = libc_mktime(2026, 3, 23, 10, 0, 0);
+  EXPECT_EQ(t.timestamp, libc_result);
+}
+
 TEST(RecalcTimestampLocal, YearBoundaryDST) {
   // Test southern hemisphere DST across year boundary
   // Australia/Sydney: DST active from October to April (spans Jan 1)