Document how to perform the inverse of (gm|local)time
Document how to perform the inverse of (gm|local)time

The goal is to make the documentation slightly more self contained on
the links between the time functions.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml