The International Earth Rotation and Reference Systems Service (IERS), based at the Paris Observatory, announces twice yearly whether or not there will be a leap second at the end of the following June or December. This announcement is made about five months beforehand. This information can be found in the Bulletins C on their website (use link above). It is possible to subscribe to this information by e-mail.

The NPL Telephone Time Service ( PDF, 81 kB) provides information on forthcoming leap seconds and a cumulative total of leap seconds. Note that a 'June' leap second occurs at 01:00 BST (1 a.m.) on July 1st in the UK, as summer time is in force and is one hour ahead of UTC (Coordinated Universal Time, the international time scale based on atomic clocks).

IERS Bulletin A, which is updated weekly, and which is also available on their website, gives a day-by-day estimate of the difference between UT1 (Universal Time 1, a more precisely defined form of Greenwich Mean Time) and UTC (as broadcast by time signals) for the coming year, together with a formula for use further ahead. When this (UT1 - UTC) figure falls below -0.35 s a positive leap second is likely to be needed soon.