The Hourglass Door by Lisa Mangum
Abby's life is going according to plan, she's dating the boy next door, she plans to be college roommates with her best friend, and her parents are throwing the same bowling birthday party for her that they always do. If life lacks a little adventure or spontaneity, well at least she knows what to expect from life. Then Dante joins their school. He's a foreign exchange student from Italy. Abby finds herself fighting a growing attraction to him. The more she gets to know him, the more mysteries she discovers. For one, time literally stops when she is around him. Then he disappears for days at a time. Who is Dante and what is he hiding?
This story is a little slow to get started, but it's worth it. It has a little bit of everything mystery, romance, time travel, and lots of surprises. There are lots of Italian phrases sprinkled through the story to give it an unique feel.