Curse of Time is a magical affliction resulted from being overwhelmed by immense Time Magic. It is one of the reasons why the use of Time Magic (with exception of at least two of its low-tier spells Time Delaying Spell and Rewind Spell) is strictly forbidden that such act deemed as terrible as Necromancy, the other being accidental creation of time loop or (possibly) alternate timelines. As shown through the case of Molly McIntyre, the victim is caught outside the flow of time, leading him/her to live an unnatural existence where memories pertaining him/her will be periodically reset and worse, gradually corrupting him/her into a monster. Even in his/her human form, he/she displayed signs of personality corruption, devoid of restraints from displaying worsts in him/her and plagued by abominable thoughts. Additionally, the victim does not age and remains that way until the curse is broken.
So far only Great Clock of Horologium Chamber which strong enough to inflict this terrible maladiction.
Breaking the Curse of Time is a difficult endeavor, as it involves separating the victim's corrupted half from his/her human half which then split into 7 fragments that contain her memories and powers — 6 of them are placed into inanimate objects associated with the victim in question (ex. Molly's Classic Witch Set) and kept separated from one another while the seventh one within the Time Void (endless space and true contain Sealed Door of Horologium Chamber). A magic seal then placed over the Chamber to keep his/her splintered corrupted half trapped as it continuously erode the curse until it is eventually broken and has a failsafe feature in form of a time loop which activates should either the Chamber or the seal got disturbed before it could complete its purpose. The seal also enables the victim's human half to continue a normal life in that meantime and, despite memories pertaining him/her in everyone still periodically reset, his/her own memories also reset as well, keeping him/her from realizing what happened and ensuring the seal to continue erode the curse without any hindrance.
At least in Molly's case, the use of Magic Keys of Horologium Chamber (with exception of Canopus Key) was involved. Golden Magic Age-era professors who attempted to break her curse had used them to hide 6 of splintered fragments of her Ghost Witch half in their respective dungeons each contained within pieces of her Classic Witch Set. The same set would later retrieved by Akko and co. 200 years later as they explore the Sealed Door's dungeons, unknowingly reunited fragments within and subsequently reformed the Ghost Witch in process.
As effective as it seemed, the procedure Golden Age of Magic-era professors of Luna Nova conducted to save Molly from the Curse of Time has some limitations and weaknesses. Owing to the great power Curse of Time came from, it took decades for the magic seal to completely break the curse. To make matter worse, as the seal is about to complete its purposes, its hold over the seventh fragment of Molly's corrupted half weakens also, allowing her human half to remember who she was and manipulated Akko and co. to reunite her missing fragments. By the time she succeeded, however, the seal has completed its purposes, allowing the gang to defeat her restored Ghost Witch form and purged the curse once and for all, restoring Molly back to normal.
- It's implied that, in addition of Horologium Clock's nigh-limitless power which proved too great for her to control (a fact she learned in harshest way possible), Molly became afflicted with Curse of Time because she unintentionally damaged the Clock when attempting to go back in time, as Akko and co. find it broken when they entered Horologium Chamber 200 years later.