A few definitions relating the Time subsystem:

Term Meaning
Ephemeris Trajectory Data
Ephemeris Time Universal Time (Absolute)
Epoch Ephemeris Time Value
J2000 12:00 Jan 1, 2000
Ephemeris Period Time (Relative)

Time representations

Many different representations of time have been defined. The default representation at JPL is et. This stands for Ephemeris Time. SPICE, and thus MaxQ, represent time as Ephemeris Time.


In astronomy Ephemeris refers to data describing the state (position and velocity) of an object over time.

Ephemeris Time

Ephemeris Time represents the time variable of a trajectory. Often Ephemeris Times are given in “Seconds past J2000”. MaxQ serializes SEphemerisTime values equivalently, and values are displayed in the Unreal Editor the same way. Many Blueprint actions accept an SEphemerisTime variable as et.

Editing an Ephemeris Time in UE.  The user sees the units, ‘seconds’

Editing an Ephemeris Time in UE. The user sees the units, ‘seconds’


A particular value for Ephemeris Time is referred to as as an Epoch.

J2000 Epoch

The J2000 epoch means “12:00 Terrestrial Time January 1, 2000”.

Ephemeris Period

In MaxQ, an Ephemeris Period is the difference in time between two Epochs. Data type SEphemerisPeriod is used for Ephemeris Periods. Internally this is serialized as “Seconds”. A number of mathematical operators allow addition of Ephemeris Periods to Ephemeris Times, etc.

MaxQ Blueprint Actions

A number of Blueprint actions are available to manipulate time variables.

Time Initializations

et Now The current time
str2et String to ET
utc2et UTC to Ephemeris Time

String-To-SEphemerisTime initializations utilize str2et but do not intrinsically handle errors (although an error may remain signaled within SPICE, causing the next SPICE call to diagnose it).

Time Conversions

scs2e SCLK string to ET
timout Time Output
sce2s ET to SCLK string
etcal Convert ET to Calendar format
et2utc Ephemeris Time to UTC
et2lst ET to Local Solar Time
lspcn Longitude of the sun, planetocentric
deltet Delta ET, ET - UTC
tpictr Create a Time Format Picture

SEphemerisTime-To-String conversions utilize et2utc. SEphemerisTime converts to/from Double types as Seconds past J2000. Similarly converting SEphemerisPeriods to/from Double types gives simply Seconds.


Blueprint Action
b1900 Besselian Date 1900.0
b1950 Besselian Date 1950.0
j1900 Julian Date of 1900.0 JAN 0.5
j1950 Julian Date of 1950.0 JAN 1.0
j2000 Julian Date of 2000 JAN 1.5
j2100 Julian Date of 2100 JAN 1.5
jyear Seconds per julian year
spd Seconds per day (Double)
day_period One day (SEphemerisPeriod)
tyear Seconds per tropical year (Double)
tyear_period One tropical year (SEphemerisPeriod)

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