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.
The Forgotten Hourglass is an in-app purchase that must be bought before it can be accessed.
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 dialogues suggest that the Little Girl remembers she had been here before and that she is reliving her memories.