Convert Julian Date to Gregorian Date

Question

I'm writing an application in Lua that calculates the sunset/sunrise, and to do this I had to convert the Gregorian date in to Julian days initially and do all the complex maths and such from there.

I've completed the hard maths, but now I need to convert the Julian date (2456495.6833865 as an example) back to a Gregorian one, complete with the time. The only code I've found that can do this only has the day, year and month, but no mention of the time (which I believe is expressed as a fraction of the day, in this case the numbers after the decimal point)

Any help would be greatly appreciated - The website below has the functionality but I cant find any code or ways of doing it:

http://ssd.jpl.nasa.gov/tc.cgi#top

Thanks again,

FYP.

Answer 1

This answer shows the way to convert from a Unix timestamp to a Julian date. This is how you'd do it in Lua:

local julian = (os.time() / 86400) + 2440587.5
print(julian)
-- 2456496.1647338

And so this is how you'd convert in the other direction:

print( (julian - 2440587.5) * 86400) )
-- 1374508633

You can then convert this into a date and time using os.date().