[intercipere]

Problem

LST is off by about 4 minutes (≈1° in RA) for |UTC offset| hours every day, whenever the local date and the UTC date differ:

• East of UTC (e.g. Europe), from local midnight until UTC midnight: LST is about +3m50s too high (e.g. 00:00–02:00 in Central European Summer Time).
• West of UTC (the Americas), every evening once the UTC date has rolled over: LST is about −4m03s too low (e.g. after 19:00/20:00 on the US East Coast, after 16:00/17:00 on the West Coast).
• Never at UTC+0.

LST and _zeroPosRA are only computed when time/date/offset/longitude are set (i.e. on connect), on :SHP and on park. So a connect, park or set-home inside that window leaves the mount ~1° off in RA until the next one. Plate-solve centering usually hides it.

Cause

Mount::calculateLst() passes the local date (getLocalDate()) together with the UTC time (getUtcTime()) to Sidereal::calculateByDateAndTime(). When the two dates differ, the formula's day count is off by one, which is one day of sidereal drift (0.985647°/day ≈ 3m56.6s).

Fix

New pure helper core::localToUtcDate(localDate, localSecondsOfDay, utcOffsetHours) in core/CalendarMath. It works out the day shift (−1/0/+1) from the local time of day minus the UTC offset and applies it with the existing core::addDays(). calculateLst() now passes that UTC date. The GPS path (calculateByGPS) already uses the UTC date and is unchanged.

Testing

• 9 new unit tests in unit_tests/test_core/test_calendar.cpp: east/west of UTC inside and outside the window, UTC+0, exactly at UTC midnight, both year boundaries, and a leap day.

• Bench, ESP32 (OAE V1), site and time set the way ASCOM/OATControl set them (:Sg/:SG/:SC/:SL), :XGL# compared with IAU GMST at longitude 0:

  Local time	Before (V1.13.18)	After
  UTC+2, 2026-10-14 01:30	+229 s	−7 s
  UTC+2, 2026-10-14 03:30 (control)	−7 s	−7 s
  UTC−5, 2026-10-13 20:00	−244 s	−7 s
  UTC−5, 2026-10-13 18:00 (control)	−7 s	−7 s
  UTC+10, 2027-01-01 05:00	+230 s	−7 s

  The remaining −7 s is a constant offset of the existing sidereal formula and not part of this change.

• Builds for oaeboardv1 and ramps.

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