Early entries for both the British Sprint and British Middle Championships 2017 close on Monday 14 August 2017 at 00.00 midnight (BST).
I'd thought this meant I could enter tomorrow, but Wikipedia says:
In 24-hour time notation, "0:00" and "0:00:00" refer to midnight at the start of a given date. Some styles, such as ISO 8601, allow 24:00 to refer to the end of a day.