diff --git a/src/timer/timer_game_calendar.h b/src/timer/timer_game_calendar.h
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--- /dev/null
+++ b/src/timer/timer_game_calendar.h
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+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file timer_game_calendar.h Definition of the game-calendar-timer */
+
+#ifndef TIMER_GAME_CALENDAR_H
+#define TIMER_GAME_CALENDAR_H
+
+/**
+ * Timer that is increased every 27ms, and counts towards ticks / days / months / years.
+ *
+ * The amount of days in a month depends on the month and year (leap-years).
+ * There are always 74 ticks in a day (and with 27ms, this makes 1 day 1.998 seconds).
+ *
+ * IntervalTimer and TimeoutTimer based on this Timer are a bit unusual, as their count is always one.
+ * You create those timers based on a transition: a new day, a new month or a new year.
+ *
+ * Additionally, you need to set a priority. To ensure deterministic behaviour, events are executed
+ * in priority. It is important that if you assign NONE, you do not use Random() in your callback.
+ * Other than that, make sure you only set one callback per priority.
+ *
+ * For example:
+ *   IntervalTimer<TimerGameCalendar>({TimerGameCalendar::DAY, TimerGameCalendar::Priority::NONE}, [](uint count){});
+ *
+ */
+class TimerGameCalendar {
+public:
+	enum Trigger {
+		DAY,
+		MONTH,
+		YEAR,
+	};
+	enum Priority {
+		NONE, ///< These timers can be executed in any order; there is no Random() in them, so order is not relevant.
+
+		/* All other may have a Random() call in them, so order is important.
+		 * For safety, you can only setup a single timer on a single priority. */
+		AUTOSAVE,
+		COMPANY,
+		DISASTER,
+		ENGINE,
+		INDUSTRY,
+		STATION,
+		SUBSIDY,
+		TOWN,
+		VEHICLE,
+	};
+
+	struct TPeriod {
+		Trigger trigger;
+		Priority priority;
+
+		TPeriod(Trigger trigger, Priority priority) : trigger(trigger), priority(priority) {}
+
+		bool operator < (const TPeriod &other) const
+		{
+			if (this->trigger != other.trigger) return this->trigger < other.trigger;
+			return this->priority < other.priority;
+		}
+
+		bool operator == (const TPeriod &other) const
+		{
+			return this->trigger == other.trigger && this->priority == other.priority;
+		}
+	};
+
+	using TElapsed = uint;
+	struct TStorage {
+	};
+};
+
+#endif /* TIMER_GAME_CALENDAR_H */