diff --git a/Core/memory.c b/Core/memory.c
index 3e7d4a4..20aaac9 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -192,9 +192,20 @@ static uint8_t read_mbc_ram(GB_gameboy_t *gb, uint16_t addr)
                 break; // Read RAM
             case 5:
                 switch (addr & 3) {
-                    case 0: return (((gb->rtc_latched.high & 7) << 8) + gb->rtc_latched.days) / 7; // Week count
-                    case 1: return gb->rtc_latched.hours |
-                            (((((gb->rtc_latched.high & 7) << 8) + gb->rtc_latched.days) % 7) << 5); // Hours and weekday
+                    case 0: { // Week count
+                        unsigned total_days = (((gb->rtc_latched.high & 7) << 8) + gb->rtc_latched.days);
+                        if (gb->rtc_latched.high & 0x20) {
+                            return total_days / 7 - 1;
+                        }
+                        return total_days / 7;
+                    }
+                    case 1: { // Week count
+                        unsigned total_days = (((gb->rtc_latched.high & 7) << 8) + gb->rtc_latched.days);
+                        if (gb->rtc_latched.high & 0x20) {
+                            return gb->rtc_latched.hours | 0xe0; // Hours and weekday
+                        }
+                        return gb->rtc_latched.hours | ((total_days % 7) << 5); // Hours and weekday
+                    }
                     case 2: return gb->rtc_latched.minutes;
                     case 3: return gb->rtc_latched.seconds;
                 }
@@ -616,7 +627,7 @@ static void write_mbc(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                         case 0x11: {
                             uint8_t flags = gb->rtc_real.high & 0xc0;
                             memcpy(&gb->rtc_real, &gb->rtc_latched, sizeof(gb->rtc_real));
-                            gb->rtc_real.high &= ~0xc0;
+                            gb->rtc_real.high &= ~0xe0;
                             gb->rtc_real.high |= flags;
                             break;
                         }
@@ -762,8 +773,12 @@ static void write_mbc_ram(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                     case 0: {
                         unsigned total_days = (((gb->rtc_latched.high & 7) << 8) + gb->rtc_latched.days);
                         total_days = total_days % 7 + value * 7;
+                        bool had_illegal_weekday = gb->rtc_latched.high & 0x20;
                         gb->rtc_latched.days = total_days;
                         gb->rtc_latched.high = total_days >> 8;
+                        if (had_illegal_weekday) {
+                            gb->rtc_latched.high |= 0x20;
+                        }
                         return;
                     }
                     case 1: {
@@ -772,6 +787,9 @@ static void write_mbc_ram(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                         gb->rtc_latched.hours = value & 0x1F;
                         gb->rtc_latched.days = total_days;
                         gb->rtc_latched.high = total_days >> 8;
+                        if ((value & 0xE0) == 0xE0) { // Illegal weekday
+                            gb->rtc_latched.high |= 0x20;
+                        }
                         return;
                     }
                     case 2: gb->rtc_latched.minutes = value; return;