Add setting for inflation dates mode

src/saveload/afterload.cpp

@@ -3831,6 +3831,10 @@ bool AfterLoadGame()
 		UpdateAllAnimatedTileSpeeds();
 	}
 
+	if (SlXvIsFeatureMissing(XSLFI_INFLATION_FIXED_DATES)) {
+		_settings_game.economy.inflation_fixed_dates = !IsSavegameVersionBefore(SLV_GS_INDUSTRY_CONTROL);
+	}
+
 	InitializeRoadGUI();
 
 	/* This needs to be done after conversion. */

src/saveload/extended_ver_sl.cpp

@@ -144,6 +144,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_NEWGRF_INFO_EXTRA,      XSCF_NULL,                1,   1, "newgrf_info_extra",         nullptr, nullptr, nullptr        },
 	{ XSLFI_INDUSTRY_CARGO_ADJ,     XSCF_IGNORABLE_UNKNOWN,   1,   1, "industry_cargo_adj",        nullptr, nullptr, nullptr        },
 	{ XSLFI_REALISTIC_TRAIN_BRAKING,XSCF_NULL,                1,   1, "realistic_train_braking",   nullptr, nullptr, "VLKA"         },
+	{ XSLFI_INFLATION_FIXED_DATES,  XSCF_IGNORABLE_ALL,       1,   1, "inflation_fixed_dates",     nullptr, nullptr, nullptr        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };
 

src/saveload/extended_ver_sl.h

@@ -98,6 +98,7 @@ enum SlXvFeatureIndex {
 	XSLFI_NEWGRF_INFO_EXTRA,                      ///< Extra NewGRF info in savegame
 	XSLFI_INDUSTRY_CARGO_ADJ,                     ///< Industry cargo adjustment patch
 	XSLFI_REALISTIC_TRAIN_BRAKING,                ///< Realistic train braking
+	XSLFI_INFLATION_FIXED_DATES,                  ///< Inflation is applied between fixed dates
 
 	XSLFI_RIFF_HEADER_60_BIT,                     ///< Size field in RIFF chunk header is 60 bit
 	XSLFI_HEIGHT_8_BIT,                           ///< Map tile height is 8 bit instead of 4 bit, but savegame version may be before this became true in trunk