(defun timer-inc-time (timer secs &optional usecs)
"Increment the time set in TIMER by SECS seconds and USECS microseconds.
-SECS may be a fraction."
+SECS may be a fraction. If USECS is omitted, that means it is zero."
(let ((time (timer-relative-time
(list (aref timer 1) (aref timer 2) (aref timer 3))
secs