Defining new unit (year, century) in siunitx

Question

I'm having a hard time figuring out the LaTeX commands to define new units of year and century in siunitx.

I'd like to write \SI{3}{\cm\per\year} and have it show: 3 cm yr-1, or \SI{42}{\micro\meter\per\century} become 4.2E-5 m/century

Answer 1

In your preamble

\DeclareSIUnit\century{century}
\DeclareSIUnit\year{yr}

See the siunitx manual (v2.1 series), Section 4.5, page 12.