I want to convert the epoch time of a [TLE](https://en.wikipedia.org/wiki/Two-line_element_set) to UTC. At this moment I am less interested in explanations and history - I just want to know quantitatively, with certainty, how to convert.

I'm confused because I see references to both UT and JD in NASA and Celestrak sites as show below. Since Julian Date currently leads UT by approximately [68.184 seconds](https://space.stackexchange.com/a/13810/12102) (as well as a 0.5D offset), I am stuck.

The epoch time in the TLE is used in at least two ways. First - it's the time when the prediction from the TLE is most accurate. Second - (I think) it is the moment in time corresponding to the mean anomaly value in line 2 of the TLE, which is 66.7566 degrees in this example:

```
TLE = """
1 25544U 98067A   16031.25992506  .00006019  00000-0  97324-4 0  9994
2 25544  51.6430  25.8646 0006733  62.5910  66.7566 15.54344726983528
"""

```

**my question:** what is the exact relationship between the epoch time in a TLE and the corresponding time in UTC?

The current difference can also be shown with [Skyfield](http://rhodesmill.org/skyfield/):

```
from skyfield.api import JulianDate

jd = JulianDate(utc=(2016,1,1))

print jd.tt, "days"
print jd.tt % 1, "day fraction"
print ((jd.tt % 1) - 0.5) * 24. * 3600, "seconds"

```

gives

```
2457388.50079 days
0.500789166894 day fraction
68.1840196252 seconds

```

this [NASA](http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/tle_def.html) page calls the fraction "Julian Day Fraction":

> 
> [![NASA TLE 2line.gif](https://i.stack.imgur.com/13u3a.gif)](https://i.stack.imgur.com/13u3a.gif)
> 
> 
> [![NASA TLE screenshot](https://i.stack.imgur.com/dYlms.png)](https://i.stack.imgur.com/dYlms.png)
> 
> 
> 

but this [Celestrak](https://celestrak.org/columns/v04n03/#FAQ03) page states that

> 
> [![Celestrak TLE screenshot](https://i.stack.imgur.com/Obez8.png)](https://i.stack.imgur.com/Obez8.png)
> 
> 
> *"Note that the epoch day starts at UT midnight (not noon) and that all times are measured mean solar rather than sidereal time units (the answer to our third question)."*
> 
> 
>