This milestone will actually take place at 5:30 BST (12:30 EST) when Queen Elizabeth II surpasses her great-great grandmother, Queen Victoria, as Britain's longest reigning monarch.
This is how the New York Times noted Queen Victoria's long reign:
Queen Victoria achieved her record on September 23, 1896. She acknowledged the event with this comment in her diary: “Today is the day on which I have reigned longer, by a day, than any English sovereign."
Victoria's record is 23,226 days, 16 hours and 23 minute. Elizabeth's achievement was calculated by assuming that her father died at 1 a.m., in the morning (actual time not known) and factoring in leap years, which brings us to today at 5:30 p.m.