Time. Usually taken for granted but it is of the utmost essence in aviation. For time and speed together relates to distance and we can calculate how long a flight will take and then estimate our time of arrival. These calculations can also provide us with our fuel consumption during the flight.
Most countries now implement some form of daylight saving, make sure to read to AIP to see when the period starts or expires, important when flying VFR and the airport closes at of just after sunset and VFR is not allowed after sunset.
Due to the rotation of the Earth, the sun rises in the east and sets in the west. Anywhere during this period we have sunlight, daylight, twilight and night. As you can imagine, this will be different on each latitude. During winter on 90° N there will be not much daylight at all; and the daylight hours between 30° N and 30° S will be about the same length throughout the whole year.
Sunlight is the period between sunrise and sunset. Sunrise is the exact time where the top part of the sun becomes visible on the horizon and sunset is where the top part of the sun disappears under the horizon.
After the sun sets below the horizon here is still a bit of daylight left and the same happens in the morning just before the sun itself is visible. These short periods are called twilight and is measured when the center of the sun is 6° below the horizon. The time the sun spends in these 6° windows varies considerably as in the tropics evening darkness sets in quite abruptly, resulting a very short period of twilight in the lower latitudes around the equator (between 30° N and 30° S).
Daylight starts when the center of disc of the sun is 6° below the horizon in the east and ends when the center of the sun is more than 6° below the horizon in the west. Thus: daylight = morning twilight + sunlight + evening twilight.
The duration of daylight depends on the season (inclination of the Earth) and the latitude the observer is on. Atmospheric conditions and altitude also have a pronounced effect on the duration of daylight, which can influence your VFR flight near the end of the day.
Most countries, during the period of March to October, have implemented some form of daylight saving where the clock is advanced one hour in March and set back again in October. This activation occurs during the night time hours, usually around 2 am in the last weekend of the month. In Europe there are discussions to put an end to this. As the energy savings were not there and most people hate it when its light outside until very late in the evening, 11 pm or so.
The sun is only straight above one meridian of longitude at a time (noon), it follows thus that: to the west from that sun position it is earlier than noon (AM) and to the east it is later (PM). For aviation purposes this can really be confusing so they decided to use UTC for flight planning purposes.
This is the actual time on a meridian of longitude and each meridian has its own local time. For example: if the LMT on 30° W is 1200 then on 15° W it is one hour later, 1300 LMT. It helps to draw a horizontal line and set 0° in the middle and divide the line from the middle to the left and right in equal parts each indicating 15° until you get to 180° E or W.
To easily calculate the time on a given meridian of longitude, draw the said line above and remember that the Earth rotates 360° in 24 hours, or 15° per hour, 1° in 4 minutes, 15' in 1 minute and 1' in 4 seconds. This helps in visualizing how to calculate LMT.
If you cross the international date line (located on 180° E or W) going on a westerly heading then you will enter the next day, and by going east you come back a day. Helpful should you wish to follow Amelia Earhart's flight path trying to discover what happened to her.
UTC is LMT on the 0° meridian (and up to 15° west) where this timezone (Z) has a zero UTC offset. UTC or Zulu time is used as the reference in aviation, all flight plans and other time stamps are related to this. It is written as follows 1310Z (HHMM).
To calculate the LMT on a certain meridian have a look at this question: what is the LMT at 90° 45'E at 0915Z ?
Compute this as follows: 90° times 4 minutes equals 360 minutes (6 hours) and 45' times 4 seconds is 180 seconds (3 minutes). Thus the answer should be 0915Z plus 0603 hours is 1518LMT on 90° 45'E.
UTC was once called GMT, Greenwich Mean Time.