diff --git a/include/constants/form_change_types.h b/include/constants/form_change_types.h
index b18e76879a..38481869bc 100644
--- a/include/constants/form_change_types.h
+++ b/include/constants/form_change_types.h
@@ -105,6 +105,14 @@
 // - No parameters
 #define FORM_CHANGE_BATTLE_GIGANTAMAX           17
 
-#define FORM_CHANGE_ITEM_USE_MULTICHOICE        18
+// Form change that activates at a certain time of day in the overworld automatically.
+// param1: time of day to check.
+// - DAY if Form change that activates in the daytime.
+// - NIGHT if Form change that activates at nighttime.
+#define FORM_CHANGE_TIME_OF_DAY                 18
+
+// Form change that depends on a multichoice (e.g. Rotom Catalog).
+// param2: multichoice list (starting at 0).
+#define FORM_CHANGE_ITEM_USE_MULTICHOICE        19
 
 #endif // GUARD_CONSTANTS_FORM_CHANGE_TYPES_H
diff --git a/src/clock.c b/src/clock.c
index 76297b4f4c..825ccc4448 100644
--- a/src/clock.c
+++ b/src/clock.c
@@ -11,9 +11,11 @@
 #include "main.h"
 #include "overworld.h"
 #include "wallclock.h"
+#include "constants/form_change_types.h"
 
 static void UpdatePerDay(struct Time *localTime);
 static void UpdatePerMinute(struct Time *localTime);
+static void FormChangeTimeUpdate();
 
 void InitTimeBasedEvents(void)
 {
@@ -69,6 +71,23 @@ static void UpdatePerMinute(struct Time *localTime)
         {
             BerryTreeTimeUpdate(minutes);
             gSaveBlock2Ptr->lastBerryTreeUpdate = *localTime;
+            FormChangeTimeUpdate();
+        }
+    }
+}
+
+static void FormChangeTimeUpdate()
+{
+    s32 i;
+    for (i = 0; i < PARTY_SIZE; i++)
+    {
+        struct Pokemon *mon = &gPlayerParty[i];
+        u16 targetSpecies = GetFormChangeTargetSpecies(mon, FORM_CHANGE_TIME_OF_DAY, 0);
+        
+        if (targetSpecies != SPECIES_NONE)
+        {
+            SetMonData(mon, MON_DATA_SPECIES, &targetSpecies);
+            CalculateMonStats(mon);
         }
     }
 }
diff --git a/src/data/pokemon/form_change_tables.h b/src/data/pokemon/form_change_tables.h
index 8156746310..ccb736bba8 100644
--- a/src/data/pokemon/form_change_tables.h
+++ b/src/data/pokemon/form_change_tables.h
@@ -315,8 +315,9 @@ static const struct FormChange sGiratinaFormChangeTable[] = {
 };
 
 static const struct FormChange sShayminFormChangeTable[] = {
-    {FORM_CHANGE_ITEM_USE, SPECIES_SHAYMIN_SKY, ITEM_GRACIDEA, DAY},
-    // {FORM_CHANGE_WITHDRAW, SPECIES_SHAYMIN_LAND},
+    {FORM_CHANGE_ITEM_USE,    SPECIES_SHAYMIN_SKY, ITEM_GRACIDEA, DAY},
+    {FORM_CHANGE_WITHDRAW,    SPECIES_SHAYMIN_LAND},
+    {FORM_CHANGE_TIME_OF_DAY, SPECIES_SHAYMIN_LAND, NIGHT},
     {FORM_CHANGE_TERMINATOR},
 };
 
@@ -407,8 +408,8 @@ static const struct FormChange sLandorusFormChangeTable[] = {
 };
 
 static const struct FormChange sKeldeoFormChangeTable[] = {
-    // {FORM_CHANGE_MOVE, SPECIES_KELDEO_RESOLUTE, MOVE_SECRET_SWORD, WHEN_LEARNED},
-    // {FORM_CHANGE_MOVE, SPECIES_KELDEO_ORDINARY, MOVE_SECRET_SWORD, WHEN_FORGOTTEN},
+    {FORM_CHANGE_MOVE, SPECIES_KELDEO_RESOLUTE, MOVE_SECRET_SWORD, WHEN_LEARNED},
+    {FORM_CHANGE_MOVE, SPECIES_KELDEO_ORDINARY, MOVE_SECRET_SWORD, WHEN_FORGOTTEN},
     {FORM_CHANGE_TERMINATOR},
 };
 
diff --git a/src/pokemon.c b/src/pokemon.c
index 3a48d1d38f..636acdf156 100644
--- a/src/pokemon.c
+++ b/src/pokemon.c
@@ -9088,6 +9088,21 @@ u16 GetFormChangeTargetSpeciesBoxMon(struct BoxPokemon *boxMon, u16 method, u32
                 case FORM_CHANGE_FAINT:
                     targetSpecies = formChanges[i].targetSpecies;
                     break;
+                case FORM_CHANGE_TIME_OF_DAY:
+                    switch (formChanges[i].param1)
+                    {
+                    case DAY:
+                        RtcCalcLocalTime();
+                        if (gLocalTime.hours >= DAY_EVO_HOUR_BEGIN && gLocalTime.hours < DAY_EVO_HOUR_END)
+                            targetSpecies = formChanges[i].targetSpecies;
+                        break;
+                    case NIGHT:
+                        RtcCalcLocalTime();
+                        if (gLocalTime.hours >= NIGHT_EVO_HOUR_BEGIN && gLocalTime.hours < NIGHT_EVO_HOUR_END)
+                            targetSpecies = formChanges[i].targetSpecies;
+                        break;
+                    }
+                    break;
                 }
             }
         }