This is an implementation of IAU 2006 Precession from IERS Tech Note 36.
Angles are in arcseconds, multiply by 1/60/60*Math.PI/180 to convert to degrees.
$$
\begin{align*}
t &= \frac{jd - jd00}{36525.0} \\
\\
\gamma &=
-0.052928" +
10.556378"t +
0.4932044"t^2-
0.00031238"t^3 -
0.000002788"t^4 +
0.000000026"t^5 \\
\phi &=
+84381.412819" -
46.811016"t +
0.0511268"t^2 +
0.00053289"t^3 -
0.00000044"t^4 -
0.0000000176"t^5 \\
\psi &=
-0.041775" +
5038.481484"t +
1.5584175"t^2 -
0.00018522"t^3 -
0.000026452"t^4 -
0.0000000148"t^5 \\
\epsilon &=
+84381.406" -
46.836769"t -
0.0001831"t^2 +
0.0020034"t^3 -
0.000000576"t^4 -
0.0000000434"t^5 \\
\\
ROT_{prec} &= ROT_x(-\epsilon) ROT_z(-\psi) ROT_x (\phi) ROT_z(\gamma) \\
\end{align*}
$$
JD - JD to Precess to.
JD00 - Starting epoch (2451545 for J2000)
\(ROT_{prec}\) - Precession matrix