<P> In order to keep the civil day aligned with the apparent movement of the Sun, a day according to Coordinated Universal Time (UTC) can include a negative or positive leap second . Therefore, although typically 86 400 SI seconds in duration, a civil day can be either 86 401 or 86 399 SI seconds long on such a day . </P> <P> Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service (IERS), which measures the Earth's rotation and determines whether a leap second is necessary . Leap seconds occur only at the end of a UTC - calculated month, and have only ever been inserted at the end of June 30 or December 31 . </P> <P> For civil purposes, a common clock time is typically defined for an entire region based on the local mean solar time at a central meridian . Such time zones began to be adopted about the middle of the 19th century when railroads with regularly occurring schedules came into use, with most major countries having adopted them by 1929 . As of 2015, throughout the world, 40 such zones are now in use: the central zone, from which all others are defined as offsets, is known as UTC ± 00, which uses Coordinated Universal Time (UTC). </P> <P> The most common convention starts the civil day at midnight: this is near the time of the lower culmination of the Sun on the central meridian of the time zone . Such a day may be referred to as a calendar day . </P>

There are 50 hours in a day meaning