Details

• Time systems are also known as time standards, time scales or time references.
• The time system indicates the physical process that determines how to measure the advance of time: atomic oscillations, the rotation of the Earth, some other astronomical phenomenon, etc.
• Choosing the appropriate time system is critical for precise time control. Errors of tens of seconds may accumulate over a few decades, or several hours over a few thousand years.
• The choice of time system is independent from the choice of calendar and the choice of time zone.
• Possible values for time systems include:
• Automaticstandard civil time, but not including leap seconds "TAI"atomic time, as measured by atomic clocks "TCB"solar system barycentric coordinate time "TCG"geocentric coordinate time "TDB","ET"solar system barycentric dynamical time, or ephemeris time "TT"terrestrial time, as measured at sea level on the Earth "UTC"coordinated universal time, including leap seconds "UT1"time proportional to the Earth's rotation angle "SmearedUTC"coordinated universal time, smearing leap seconds over the previous day

Examples

Basic Examples  (3)

Define an instant in the UTC time system:

Convert to atomic time, which is currently 37 leap seconds ahead of UTC:

Convert a UTC date into a Julian date in terrestrial time:

Compare with a conversion to a Julian date in UTC time:

The difference on that date was 64.184 seconds:

These are different instants of time, separated by 37 leap seconds:

Scope  (3)

Specify different time systems in a DateObject expression:

Use different time systems in combination with different calendars:

The "TDB" time system is also known as ephemeris time "ET":

Converting to the "ET" system does not change the numeric date:

Applications  (1)

Explore the transition during the last added leap second, at the end of year 2016:

Properties & Relations  (2)

Use the TimeSystem option to specify the time system of a DateObject expression:

Use TimeSystemConvert to change that date to a different time system representation:

UT1 time for a given time zone corresponds to mean solar time at the corresponding central meridian:

Neat Examples  (1)

Between 1960 and 1972, "UTC" frequently added fractional seconds and was more closely aligned to "UT1":

Starting in 1972, "UTC" adds only whole leap seconds, as needed, to remain within 1 second of "UT1":

The difference between "TAI" and "UTC" shows the number of leap seconds added since "UTC" began: