The Forgotten Hourglass is an item that is accessible when the player reaches the end of a playthrough. It is located atop the piano in Alice's Room and is used to replay the game from the beginning. It can be selected either from Alice's Room or the collections screen. Upon selection, a prompt displaying the current playthrough count will appear to confirm the selection.
In Forgotten Hourglass playthroughs, everything previously unlocked/ purchased will remain. There are further unlockables on subsequent playthroughs and there are several differences compared to the Main Story playthrough.
- There are 2 new accessible rooms. From the Basement, the left path leads to the Mask Room: a secret room of the Masked Lady. The Balcony's telescope is now a path leading to the Sakura Forest: a forest filled with cherry blossom trees (reminiscent of previously unlocking Sakura iro no yume).
- Moments (still images, unlike Memories) are now available for unlocking and viewing in the Library or Mask Room.
- As the game progresses, players can unlock songs in the two new collections, Samsara Collection and Etude Collection that contain 25 songs all-in-all, and 12 Extra difficulty charts of certain existing songs.
- Players reach the game's full completion at the end of playthrough 4 and no unlockables are left. After this, players can replay the Main Story by selecting the Forgotten Hourglass (the prompt options are different). In these playthroughs, rooms are reverted to their original appearances. The Mask Room and Sakura Forest are also inaccessible. Replaying the Main Story still contributes to the playthrough counter.
- The locations of unlockables remain unchanged but different items are obtained. This also applies to what is unlocked after each Memory.
- Dialogues are different when selecting the Masked Lady, the Little Girl and other furniture in the rooms. However, the Memories remain the same.
- The rooms in the building are brighter and filled with greenery. The Piano Room's floor is filled with water. Players can view Moments in the Library. The Attic's ceiling is decorated with sparkling crystals and outside the window, the fog has cleared. The sky at the Top Floor depicts day instead of night.
- The song requirements for unlocking The New Tree and Goodbye Memories are also changed: For The New Tree, instead of clearing Suspenseful Third Day (Easy), clear The 105th Days (Normal) in playthrough 2 or The Way Home (Normal) in playthrough 3 and 4. For Goodbye, instead of clearing Fluquor (Easy), clear Fluquor (Normal) in playthrough 2 and 3 or Marigold (Normal) in playthrough 4.
- The Forgotten Hourglass is an in-app purchase that must be bought before it can be accessed.
- The dialogues suggest that the Little Girl remembers she had been here before and that she is reliving her memories.