A day is a unit of time. If you add the unit year and month, you get a date.
Over time the average length of the day is getting longer as the Earth slows. As a result, the length of a solar day in 2012 is slightly longer than 86400 seconds.
Therefore, A leap-second may be added or removed depending on the Earth's rotational changes. A day in UTC may have 86399 seconds or 86401 seconds where necessary in order to keep the day aligned with the Sun.
The length of the solar day is the standard way that humans measure time.
A day has traditionally been subdivided into:
If you choose the time zone United States, Los Angeles, the beginning and end of each day is calculated based on Los Angeles Time, even if the data comes from another time zone such as New York, London, or Moscow.