Emma Trevayne - The House of Months and Years

A girl must stop the Boogeyman living in her home from stealing her family's warmest memories in this "eerie and enchanting story" (Publishers Weekly) from the author of Flights and Chimes and Mysterious Times and The Accidental Afterlife of Thomas Marsden.
When her distant aunt and uncle die, Amelia Howling is forced to move into their home when they leave her parents in charge of their children. Her parents assure her that it will be like having a grand adventure with three new siblings, but Amelia is not convinced. Luckily, the house is large, filled with nooks and crannies perfect for hiding from her cousins.

But even with all the nooks and crannies, the rumbling and crumbling rooms are more sinister than they seem. The house was built years ago by a creature named Horatio, and he's been waiting for the perfect human inhabitant: Amelia. Horatio has the power to travel through time and memories, and lures Amelia into his world. The memories of children, he told her, were the best, and Amelia agreed-her cousins were full of good memories. Until she noticed that once she and Horatio visited a memory, it was gone forever. And she had been stealing the good memories of her cousins and their parents without even noticing!

Horrified and scared, Amelia lets her cousins in on her secret, and asks them for help. Together, they must race through time to recover their minds and break the perfect clockwork of the evil Calendar House.