diff --git a/src/cheat_gui.cpp b/src/cheat_gui.cpp
--- a/src/cheat_gui.cpp
+++ b/src/cheat_gui.cpp
@@ -12,7 +12,6 @@
 #include "cheat_type.h"
 #include "company_base.h"
 #include "company_func.h"
-#include "date_func.h"
 #include "saveload/saveload.h"
 #include "vehicle_base.h"
 #include "textbuf_gui.h"
@@ -108,9 +107,9 @@ static int32 ClickChangeDateCheat(int32 
 	new_value = Clamp(new_value, MIN_YEAR, MAX_YEAR);
 	if (new_value == TimerGameCalendar::year) return TimerGameCalendar::year;
 
-	YearMonthDay ymd;
-	ConvertDateToYMD(TimerGameCalendar::date, &ymd);
-	TimerGameCalendar::Date new_date = ConvertYMDToDate(new_value, ymd.month, ymd.day);
+	TimerGameCalendar::YearMonthDay ymd;
+	TimerGameCalendar::ConvertDateToYMD(TimerGameCalendar::date, &ymd);
+	TimerGameCalendar::Date new_date = TimerGameCalendar::ConvertYMDToDate(new_value, ymd.month, ymd.day);
 
 	/* Shift cached dates before we change the date. */
 	for (auto v : Vehicle::Iterate()) v->ShiftDates(new_date - TimerGameCalendar::date);
@@ -323,7 +322,7 @@ struct CheatWindow : Window {
 					switch (ce->str) {
 						/* Display date for change date cheat */
 						case STR_CHEAT_CHANGE_DATE:
-							SetDParam(0, ConvertYMDToDate(MAX_YEAR, 11, 31));
+							SetDParam(0, TimerGameCalendar::ConvertYMDToDate(MAX_YEAR, 11, 31));
 							width = std::max(width, GetStringBoundingBox(ce->str).width);
 							break;