use default timezone based on (longitude / 15) if not specified

src/cpp/fs/ArgumentParser.cpp

@@ -524,7 +524,7 @@ MainArgumentParser::MainArgumentParser(const int argc, const char* const argv[])
       settings.fuel_lookup, "--fuel-lut", "Use specified fuel lookup table", false
     );
     register_setter<
-      DurationSize>(settings.utc_offset, "--tz", "UTC offset (hours)", true, &parse_value<DurationSize>);
+      DurationSize>(settings.utc_offset, "--tz", "UTC offset (hours)", false, &parse_value<DurationSize>);
     register_setter<size_t>(
       [&](const auto v) { settings.static_curing = v; },
       "--curing",

src/cpp/fs/Settings.h

@@ -145,7 +145,7 @@ class Settings
   // Minimum Fine Fuel Moisture Code required for spread during the night
   MathSize minimum_ffmc_at_night{0.0};
   // Offset from UTC to use for entire simulation (hours)
-  DurationSize utc_offset{0.0};
+  std::optional<DurationSize> utc_offset{};
   // Offset from sunrise at which the day is considered to start (hours)
   DurationSize offset_sunrise{0.0};
   // Offset from sunrise at which the day is considered to end (hours)

src/cpp/fs/StartPoint.cpp

@@ -94,12 +94,28 @@ static array<tuple<DurationSize, DurationSize>, MAX_DAYS> make_days(
   static const Settings& settings = settings::instance();
   array<tuple<DurationSize, DurationSize>, MAX_DAYS> days{};
   array<DurationSize, MAX_DAYS> day_length_hours{};
+  constexpr auto DEGREES_PER_TZ_HOUR = 15.0;
+  const auto tz_calculated = floor(longitude / DEGREES_PER_TZ_HOUR);
+  auto utc_offset = settings.utc_offset.value_or(tz_calculated);
+  if (!settings.utc_offset.has_value())
+  {
+    logging::note("Using calculated timezone offset of ({:0.1g}) since not specified", utc_offset);
+  }
+  else if (1 < abs(tz_calculated - utc_offset))
+  {
+    logging::warning(
+      "Specified timezone ({:0.1g}) differs from calculated timezone ({:0.1g}) by {:0.1g} hours",
+      utc_offset,
+      tz_calculated,
+      (tz_calculated - utc_offset)
+    );
+  }
   for (size_t i = 0; i < day_length_hours.size(); ++i)
   {
     const auto [sunrise, sunset] = sunrise_sunset(static_cast<int>(i), latitude, longitude);
     days[i] = make_tuple(
-      fix_hours(sunrise + settings.utc_offset + settings.offset_sunrise),
-      fix_hours(sunset + settings.utc_offset - settings.offset_sunset)
+      fix_hours(sunrise + utc_offset + settings.offset_sunrise),
+      fix_hours(sunset + utc_offset - settings.offset_sunset)
     );
     day_length_hours[i] = get<1>(days[i]) - get<0>(days[i]);
   }