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()
.