diff --git a/src/timer/timer_game_common.cpp b/src/timer/timer_game_common.cpp
new file mode 100644
--- /dev/null
+++ b/src/timer/timer_game_common.cpp
@@ -0,0 +1,140 @@
+/*
+ * 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 .
+ */
+
+/**
+ * @file timer_game_common.cpp
+ * This file implements the common timer logic for the game-calendar timers.
+ */
+
+#include "../stdafx.h"
+
+#include "timer_game_common.h"
+#include "timer_game_calendar.h"
+#include "timer_game_economy.h"
+
+#include "../safeguards.h"
+
+#define M(a, b) ((a << 5) | b)
+static constexpr uint16_t _month_date_from_year_day[] = {
+ M(0, 1), M(0, 2), M(0, 3), M(0, 4), M(0, 5), M(0, 6), M(0, 7), M(0, 8), M(0, 9), M(0, 10), M(0, 11), M(0, 12), M(0, 13), M(0, 14), M(0, 15), M(0, 16), M(0, 17), M(0, 18), M(0, 19), M(0, 20), M(0, 21), M(0, 22), M(0, 23), M(0, 24), M(0, 25), M(0, 26), M(0, 27), M(0, 28), M(0, 29), M(0, 30), M(0, 31),
+ M(1, 1), M(1, 2), M(1, 3), M(1, 4), M(1, 5), M(1, 6), M(1, 7), M(1, 8), M(1, 9), M(1, 10), M(1, 11), M(1, 12), M(1, 13), M(1, 14), M(1, 15), M(1, 16), M(1, 17), M(1, 18), M(1, 19), M(1, 20), M(1, 21), M(1, 22), M(1, 23), M(1, 24), M(1, 25), M(1, 26), M(1, 27), M(1, 28), M(1, 29),
+ M(2, 1), M(2, 2), M(2, 3), M(2, 4), M(2, 5), M(2, 6), M(2, 7), M(2, 8), M(2, 9), M(2, 10), M(2, 11), M(2, 12), M(2, 13), M(2, 14), M(2, 15), M(2, 16), M(2, 17), M(2, 18), M(2, 19), M(2, 20), M(2, 21), M(2, 22), M(2, 23), M(2, 24), M(2, 25), M(2, 26), M(2, 27), M(2, 28), M(2, 29), M(2, 30), M(2, 31),
+ M(3, 1), M(3, 2), M(3, 3), M(3, 4), M(3, 5), M(3, 6), M(3, 7), M(3, 8), M(3, 9), M(3, 10), M(3, 11), M(3, 12), M(3, 13), M(3, 14), M(3, 15), M(3, 16), M(3, 17), M(3, 18), M(3, 19), M(3, 20), M(3, 21), M(3, 22), M(3, 23), M(3, 24), M(3, 25), M(3, 26), M(3, 27), M(3, 28), M(3, 29), M(3, 30),
+ M(4, 1), M(4, 2), M(4, 3), M(4, 4), M(4, 5), M(4, 6), M(4, 7), M(4, 8), M(4, 9), M(4, 10), M(4, 11), M(4, 12), M(4, 13), M(4, 14), M(4, 15), M(4, 16), M(4, 17), M(4, 18), M(4, 19), M(4, 20), M(4, 21), M(4, 22), M(4, 23), M(4, 24), M(4, 25), M(4, 26), M(4, 27), M(4, 28), M(4, 29), M(4, 30), M(4, 31),
+ M(5, 1), M(5, 2), M(5, 3), M(5, 4), M(5, 5), M(5, 6), M(5, 7), M(5, 8), M(5, 9), M(5, 10), M(5, 11), M(5, 12), M(5, 13), M(5, 14), M(5, 15), M(5, 16), M(5, 17), M(5, 18), M(5, 19), M(5, 20), M(5, 21), M(5, 22), M(5, 23), M(5, 24), M(5, 25), M(5, 26), M(5, 27), M(5, 28), M(5, 29), M(5, 30),
+ M(6, 1), M(6, 2), M(6, 3), M(6, 4), M(6, 5), M(6, 6), M(6, 7), M(6, 8), M(6, 9), M(6, 10), M(6, 11), M(6, 12), M(6, 13), M(6, 14), M(6, 15), M(6, 16), M(6, 17), M(6, 18), M(6, 19), M(6, 20), M(6, 21), M(6, 22), M(6, 23), M(6, 24), M(6, 25), M(6, 26), M(6, 27), M(6, 28), M(6, 29), M(6, 30), M(6, 31),
+ M(7, 1), M(7, 2), M(7, 3), M(7, 4), M(7, 5), M(7, 6), M(7, 7), M(7, 8), M(7, 9), M(7, 10), M(7, 11), M(7, 12), M(7, 13), M(7, 14), M(7, 15), M(7, 16), M(7, 17), M(7, 18), M(7, 19), M(7, 20), M(7, 21), M(7, 22), M(7, 23), M(7, 24), M(7, 25), M(7, 26), M(7, 27), M(7, 28), M(7, 29), M(7, 30), M(7, 31),
+ M(8, 1), M(8, 2), M(8, 3), M(8, 4), M(8, 5), M(8, 6), M(8, 7), M(8, 8), M(8, 9), M(8, 10), M(8, 11), M(8, 12), M(8, 13), M(8, 14), M(8, 15), M(8, 16), M(8, 17), M(8, 18), M(8, 19), M(8, 20), M(8, 21), M(8, 22), M(8, 23), M(8, 24), M(8, 25), M(8, 26), M(8, 27), M(8, 28), M(8, 29), M(8, 30),
+ M(9, 1), M(9, 2), M(9, 3), M(9, 4), M(9, 5), M(9, 6), M(9, 7), M(9, 8), M(9, 9), M(9, 10), M(9, 11), M(9, 12), M(9, 13), M(9, 14), M(9, 15), M(9, 16), M(9, 17), M(9, 18), M(9, 19), M(9, 20), M(9, 21), M(9, 22), M(9, 23), M(9, 24), M(9, 25), M(9, 26), M(9, 27), M(9, 28), M(9, 29), M(9, 30), M(9, 31),
+ M(10, 1), M(10, 2), M(10, 3), M(10, 4), M(10, 5), M(10, 6), M(10, 7), M(10, 8), M(10, 9), M(10, 10), M(10, 11), M(10, 12), M(10, 13), M(10, 14), M(10, 15), M(10, 16), M(10, 17), M(10, 18), M(10, 19), M(10, 20), M(10, 21), M(10, 22), M(10, 23), M(10, 24), M(10, 25), M(10, 26), M(10, 27), M(10, 28), M(10, 29), M(10, 30),
+ M(11, 1), M(11, 2), M(11, 3), M(11, 4), M(11, 5), M(11, 6), M(11, 7), M(11, 8), M(11, 9), M(11, 10), M(11, 11), M(11, 12), M(11, 13), M(11, 14), M(11, 15), M(11, 16), M(11, 17), M(11, 18), M(11, 19), M(11, 20), M(11, 21), M(11, 22), M(11, 23), M(11, 24), M(11, 25), M(11, 26), M(11, 27), M(11, 28), M(11, 29), M(11, 30), M(11, 31),
+};
+#undef M
+
+enum DaysTillMonth {
+ ACCUM_JAN = 0,
+ ACCUM_FEB = ACCUM_JAN + 31,
+ ACCUM_MAR = ACCUM_FEB + 29,
+ ACCUM_APR = ACCUM_MAR + 31,
+ ACCUM_MAY = ACCUM_APR + 30,
+ ACCUM_JUN = ACCUM_MAY + 31,
+ ACCUM_JUL = ACCUM_JUN + 30,
+ ACCUM_AUG = ACCUM_JUL + 31,
+ ACCUM_SEP = ACCUM_AUG + 31,
+ ACCUM_OCT = ACCUM_SEP + 30,
+ ACCUM_NOV = ACCUM_OCT + 31,
+ ACCUM_DEC = ACCUM_NOV + 30,
+};
+
+/** Number of days to pass from the first day in the year before reaching the first of a month. */
+static constexpr uint16_t _accum_days_for_month[] = {
+ ACCUM_JAN, ACCUM_FEB, ACCUM_MAR, ACCUM_APR,
+ ACCUM_MAY, ACCUM_JUN, ACCUM_JUL, ACCUM_AUG,
+ ACCUM_SEP, ACCUM_OCT, ACCUM_NOV, ACCUM_DEC,
+};
+
+/**
+ * Converts a Date to a Year, Month & Day.
+ * @param date the date to convert from
+ * @returns YearMonthDay representation of the Date.
+ */
+template
+/* static */ typename TimerGame::YearMonthDay TimerGame::ConvertDateToYMD(Date date)
+{
+ /* Year determination in multiple steps to account for leap
+ * years. First do the large steps, then the smaller ones.
+ */
+
+ /* There are 97 leap years in 400 years */
+ Year yr = 400 * (date.base() / (TimerGameConst::DAYS_IN_YEAR * 400 + 97));
+ int rem = date.base() % (TimerGameConst::DAYS_IN_YEAR * 400 + 97);
+
+ if (rem >= TimerGameConst::DAYS_IN_YEAR * 100 + 25) {
+ /* There are 25 leap years in the first 100 years after
+ * every 400th year, as every 400th year is a leap year */
+ yr += 100;
+ rem -= TimerGameConst::DAYS_IN_YEAR * 100 + 25;
+
+ /* There are 24 leap years in the next couple of 100 years */
+ yr += 100 * (rem / (TimerGameConst::DAYS_IN_YEAR * 100 + 24));
+ rem = (rem % (TimerGameConst::DAYS_IN_YEAR * 100 + 24));
+ }
+
+ if (!IsLeapYear(yr) && rem >= TimerGameConst::DAYS_IN_YEAR * 4) {
+ /* The first 4 year of the century are not always a leap year */
+ yr += 4;
+ rem -= TimerGameConst::DAYS_IN_YEAR * 4;
+ }
+
+ /* There is 1 leap year every 4 years */
+ yr += 4 * (rem / (TimerGameConst::DAYS_IN_YEAR * 4 + 1));
+ rem = rem % (TimerGameConst::DAYS_IN_YEAR * 4 + 1);
+
+ /* The last (max 3) years to account for; the first one
+ * can be, but is not necessarily a leap year */
+ while (rem >= (IsLeapYear(yr) ? TimerGameConst::DAYS_IN_LEAP_YEAR : TimerGameConst::DAYS_IN_YEAR)) {
+ rem -= IsLeapYear(yr) ? TimerGameConst::DAYS_IN_LEAP_YEAR : TimerGameConst::DAYS_IN_YEAR;
+ yr++;
+ }
+
+ /* Skip the 29th of February in non-leap years */
+ if (!IsLeapYear(yr) && rem >= ACCUM_MAR - 1) rem++;
+
+ uint16_t x = _month_date_from_year_day[rem];
+
+ YearMonthDay ymd;
+ ymd.year = yr;
+ ymd.month = x >> 5;
+ ymd.day = x & 0x1F;
+ return ymd;
+}
+
+/**
+ * Converts a tuple of Year, Month and Day to a Date.
+ * @param year is a number between 0..MAX_YEAR
+ * @param month is a number between 0..11
+ * @param day is a number between 1..31
+ */
+template
+/* static */ typename TimerGame::Date TimerGame::ConvertYMDToDate(Year year, Month month, Day day)
+{
+ /* Day-offset in a leap year */
+ int days = _accum_days_for_month[month] + day - 1;
+
+ /* Account for the missing of the 29th of February in non-leap years */
+ if (!IsLeapYear(year) && days >= ACCUM_MAR) days--;
+
+ return DateAtStartOfYear(year) + days;
+}
+
+/* Create instances of the two template variants that we have.
+ * This is needed, as this templated functions are not in a header-file. */
+template TimerGame::YearMonthDay TimerGame::ConvertDateToYMD(Date date);
+template TimerGame::YearMonthDay TimerGame::ConvertDateToYMD(Date date);
+
+template TimerGame::Date TimerGame::ConvertYMDToDate(Year year, Month month, Day day);
+template TimerGame::Date TimerGame::ConvertYMDToDate(Year year, Month month, Day day);