Hilton's 24 hour cancelation policy means you must cancel the reservation no later than the property's check in time one day prior to your scheduled arrival. So, if you have a reservation for June 15th at a property with a 3 p.m. check in time, you can cancel without penalty until 2:59 p.m. (hotel time) on June 14th.

They have a separate (and about equally common) policy which allows cancelation until 11:59 p.m. (local hotel time) one day prior to scheduled arrival. Under this arrangement, if you have a reservation for June 15th, you can cancel without penalty until 11:59 p.m. (hotel time) on June 14th.

Be sure you know which you have.