fix: use UTC date for LST calculation

src/Mount.cpp

@@ -6,6 +6,7 @@
 #include "EndSwitches.hpp"
 #include "Mount.hpp"
 #include "Sidereal.hpp"
+#include "core/CalendarMath.hpp"
 #include "libs/MappedDict/MappedDict.hpp"
 
 PUSH_NO_WARNINGS
@@ -4239,13 +4240,16 @@ DayTime Mount::calculateLst()
 {
     DayTime timeUTC     = getUtcTime();
     LocalDate localDate = getLocalDate();
-    DayTime lst = Sidereal::calculateByDateAndTime(longitude().getTotalHours(), localDate.year, localDate.month, localDate.day, &timeUTC);
+    // The sidereal formula needs the UTC date, which differs from the local date for |UTC offset| hours each day.
+    core::CalendarDate utcDate
+        = core::localToUtcDate({localDate.year, localDate.month, localDate.day}, getLocalTime().getTotalSeconds(), _localUtcOffset);
+    DayTime lst = Sidereal::calculateByDateAndTime(longitude().getTotalHours(), utcDate.year, utcDate.month, utcDate.day, &timeUTC);
     // LOG(DEBUG_INFO,
-    //     "[MOUNT]: Calculating LST. UTC time: %s. Date: %d-%d-%d. Longitude: %s",
+    //     "[MOUNT]: Calculating LST. UTC time: %s. UTC date: %d-%d-%d. Longitude: %s",
     //     timeUTC.ToString(),
-    //     localDate.year,
-    //     localDate.month,
-    //     localDate.day,
+    //     utcDate.year,
+    //     utcDate.month,
+    //     utcDate.day,
     //     longitude().ToString());
     // LOG(DEBUG_INFO, "[MOUNT]: LST is: %s", lst.ToString());
     return lst;

src/core/CalendarMath.cpp

@@ -64,4 +64,22 @@ CalendarDate addDays(CalendarDate date, long days)
     return date;
 }
 
+CalendarDate localToUtcDate(CalendarDate localDate, long localSecondsOfDay, int utcOffsetHours)
+{
+    long utcSeconds = localSecondsOfDay - utcOffsetHours * 3600L;
+    long dayShift   = 0;
+    while (utcSeconds < 0)
+    {
+        utcSeconds += 86400L;
+        dayShift--;
+    }
+    while (utcSeconds >= 86400L)
+    {
+        utcSeconds -= 86400L;
+        dayShift++;
+    }
+
+    return addDays(localDate, dayShift);
+}
+
 }  // namespace core

src/core/CalendarMath.hpp

@@ -14,4 +14,9 @@ struct CalendarDate {
 /// Handles month-length tables and leap-year logic including century rules.
 CalendarDate addDays(CalendarDate date, long days);
 
+/// Convert a local calendar date to the UTC calendar date.
+/// localSecondsOfDay is the local time of day in seconds since local midnight (0..86399).
+/// utcOffsetHours is the local offset from UTC (local = UTC + offset).
+CalendarDate localToUtcDate(CalendarDate localDate, long localSecondsOfDay, int utcOffsetHours);
+
 }  // namespace core

unit_tests/test_core/test_calendar.cpp

@@ -101,3 +101,83 @@ TEST(CalendarMathTest, NegativeDays)
     EXPECT_EQ(12, result.month);
     EXPECT_EQ(31, result.day);
 }
+
+TEST(CalendarMathTest, LocalToUtcDateEastAfterMidnight)
+{
+    // UTC+2, local 2026-10-14 01:30 is UTC 2026-10-13 23:30
+    CalendarDate result = core::localToUtcDate({2026, 10, 14}, 1 * 3600L + 30 * 60L, 2);
+    EXPECT_EQ(2026, result.year);
+    EXPECT_EQ(10, result.month);
+    EXPECT_EQ(13, result.day);
+}
+
+TEST(CalendarMathTest, LocalToUtcDateEastSameDay)
+{
+    // UTC+2, local 2026-10-14 03:30 is UTC 2026-10-14 01:30
+    CalendarDate result = core::localToUtcDate({2026, 10, 14}, 3 * 3600L + 30 * 60L, 2);
+    EXPECT_EQ(2026, result.year);
+    EXPECT_EQ(10, result.month);
+    EXPECT_EQ(14, result.day);
+}
+
+TEST(CalendarMathTest, LocalToUtcDateWestEvening)
+{
+    // UTC-5, local 2026-01-15 20:00 is UTC 2026-01-16 01:00
+    CalendarDate result = core::localToUtcDate({2026, 1, 15}, 20 * 3600L, -5);
+    EXPECT_EQ(2026, result.year);
+    EXPECT_EQ(1, result.month);
+    EXPECT_EQ(16, result.day);
+}
+
+TEST(CalendarMathTest, LocalToUtcDateWestSameDay)
+{
+    // UTC-5, local 2026-01-15 18:59:59 is UTC 2026-01-15 23:59:59
+    CalendarDate result = core::localToUtcDate({2026, 1, 15}, 18 * 3600L + 59 * 60L + 59, -5);
+    EXPECT_EQ(2026, result.year);
+    EXPECT_EQ(1, result.month);
+    EXPECT_EQ(15, result.day);
+}
+
+TEST(CalendarMathTest, LocalToUtcDateUtcZero)
+{
+    CalendarDate result = core::localToUtcDate({2026, 10, 14}, 0, 0);
+    EXPECT_EQ(14, result.day);
+    result = core::localToUtcDate({2026, 10, 14}, 86399L, 0);
+    EXPECT_EQ(14, result.day);
+}
+
+TEST(CalendarMathTest, LocalToUtcDateEastYearBoundary)
+{
+    // UTC+10, local 2027-01-01 05:00 is UTC 2026-12-31 19:00
+    CalendarDate result = core::localToUtcDate({2027, 1, 1}, 5 * 3600L, 10);
+    EXPECT_EQ(2026, result.year);
+    EXPECT_EQ(12, result.month);
+    EXPECT_EQ(31, result.day);
+}
+
+TEST(CalendarMathTest, LocalToUtcDateWestLeapDay)
+{
+    // UTC-8, local 2028-02-28 17:00 is UTC 2028-02-29 01:00
+    CalendarDate result = core::localToUtcDate({2028, 2, 28}, 17 * 3600L, -8);
+    EXPECT_EQ(2028, result.year);
+    EXPECT_EQ(2, result.month);
+    EXPECT_EQ(29, result.day);
+}
+
+TEST(CalendarMathTest, LocalToUtcDateWestYearBoundary)
+{
+    // UTC-12, local 2026-12-31 12:00 is UTC 2027-01-01 00:00
+    CalendarDate result = core::localToUtcDate({2026, 12, 31}, 12 * 3600L, -12);
+    EXPECT_EQ(2027, result.year);
+    EXPECT_EQ(1, result.month);
+    EXPECT_EQ(1, result.day);
+}
+
+TEST(CalendarMathTest, LocalToUtcDateExactUtcMidnight)
+{
+    // UTC+2, local 02:00 is exactly UTC 00:00 on the same date; 01:59:59 is still the day before
+    CalendarDate result = core::localToUtcDate({2026, 10, 14}, 2 * 3600L, 2);
+    EXPECT_EQ(14, result.day);
+    result = core::localToUtcDate({2026, 10, 14}, 2 * 3600L - 1, 2);
+    EXPECT_EQ(13, result.day);
+}