In most cases, nil is assumed to be the current time. `time-subtract' is doing that here.

It could be "fixed" so

diff --git i/lisp/calendar/time-date.el w/lisp/calendar/time-date.el
index bbdcaa4db4..27ce10d7d6 100644
--- i/lisp/calendar/time-date.el
+++ w/lisp/calendar/time-date.el
@@ -200,5 +200,5 @@ time-since
     ;; Convert date strings to internal time.
     (setq time (date-to-time time)))
-  (time-subtract nil time))
+  (time-subtract nil (or time (current-time))))
 
 ;;;###autoload

but that would make the result inconsistent:

ELISP> (time-since nil)
(0 0 0 0)

ELISP> (time-since nil)
(0 0 1 0)

I'm not sure that's better. And you can always do 

(time-since (or time (current-time)))