#+title: lisp timestamp

In [[file:20200713103540-emacs_lisp.org][Emacs Lisp]] at least, times are stored as lists like so:

#+begin_src emacs-lisp
'(24814 288)
#+end_src

in the form of =(ticks . hz)=. If =hz= is =1000000000= (that's $1^{10}$), this represents a
nanosecond resolution clock.

Alternatively, if it's a list of four elements, like so:

#+begin_src emacs-lisp
'(24814 232 507836 148000)
#+end_src

This represents =(high low micro pico)=, which, in seconds, can be represented
as:

$high * 2^{16} + low + micro * 10^{-6} + pico * 10^{-12}$

or, as an [[file:20210307153354-s_expression.org][s-expression]]:

#+begin_src emacs-lisp
(defun calc-time (high low micro pico)
  (+ (* high (expt 2 16)) low (* micro (expt 10 -6)) (* pico (expt 10 -12))))

(let* ((now (current-time))
       (float-time-from-now (float-time now))
       (calc-time-from-now (apply 'calc-time now)))
  (message "float-time\t%s\ncalc-time\t%s" float-time-from-now calc-time-from-now))
#+end_src

#+RESULTS:
: float-time	1626211147.6277604
: calc-time	1626211147.6277604