java Math.atan( Math.tan() ) useless?

java Math.atan( Math.tan() ) useless?

On windows 7, look at the file
c:\windows\winsxs...\weather.js
There is a function computeSunRiseSunSet(Latitude, Longitude, TimeZone,
Year, Month, Day).
They don't cite the source of the algorithm.
One line of code is
var C2=RD*(Math.atan(Math.tan(L0+C)) - Math.atan(.9175*Math.tan(L0+C))-C);
Why is there Math.atan( Math.tan( L0+C )) ?
Is it the same as ( L0+C ) or there are corner cases ?