A day is approximately the period of time during which the Earth completes one rotation around its axis. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times. Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis. The unit of measurement "day" is defined as 86,400 SI seconds and symbolized d. The second is designated the SI base unit of time. Previously, it was defined in terms of the orbital motion of the Earth in the year 1900, but since 1967 the second and so the day are defined by atomic electron transition. A civil day is usually 24 hours, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.