Time references

Time references

Universal Time (UT)

Universal Time, or UTC (Coordinated Universal Time), or GMT (Greenwich Meridian Time). It is the basis of legal time and is derived from International Atomic Time. The difference between UT and UTC is slight, approximately one second per day. This time is the only one independant from all the geographical locations worldwide. You can use the following link to get the UT time from the U.S. Naval Observatory Master Clock.

Legal Time (LT)

Legal Time. Other names: Local Time, Real Time, Standard Time, Civil Time, and Local Clock Time. It is the time used legally in a given country. It is given by:
LT = UTC + DST + Time Zone.
DST stands for Daylight Saving Time, also named Summer Time, which is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn. [extract from Wikipedia]
Several applets are available online to know the current LT of a city. For instance, this link for travellers is a map of LT updated in real time for a certain number of cities over the world.

Mean Solar Time (MST)

Mean Solar Time. Expression:
MST = UTC + longitude(°)*24(hours)/360(°)
For instance, in Nice, we are approximately 7° east. This corresponds approximately to an additional 30 minutes compared to UT. This time reference indicates that in average, the sun is located at the zenith of every point all over the world at noon.

True Solar Time (TST)

True Solar Time. With the previous expression of MST, a slight sinusoidal fluctuation of the zenith position of the sun is observed along the year. This is due to the inclination of the axis of rotation of the earth compared to the south/north direction.
In practice, we can deduce the TST value from MST thanks to the following expression:
let d be the number of the day in the year ([1-365 or 366]), and if:
j = 2*d*pi/365.2422
TST - MST = -0.128*sin(j-0.04887)-0.165*sin(2j+0.34383)