diff --git a/syndicate/drivers/timer.rkt b/syndicate/drivers/timer.rkt
index 82c7e75..f8d3af5 100644
--- a/syndicate/drivers/timer.rkt
+++ b/syndicate/drivers/timer.rkt
@@ -32,6 +32,8 @@
                    (make-heap (lambda (t1 t2) (<= (pending-timer-deadline t1)
                                                   (pending-timer-deadline t2)))))
 
+                 (define timers (make-hash))
+
                  (define (next-timer)
                    (and (positive? (heap-count heap))
                         (heap-min heap)))
@@ -42,16 +44,27 @@
                      (when (positive? (heap-count heap))
                        (let ((m (heap-min heap)))
                          (when (<= (pending-timer-deadline m) now)
-                           (begin (heap-remove-min! heap)
-                                  (log-syndicate/drivers/timer-debug "expired timer ~a"
-                                                                     (pending-timer-label m))
-                                  (ground-send! (timer-expired (pending-timer-label m) now))
-                                  (set! count-fired (+ count-fired 1))
-                                  (loop))))))
+                           (define label (pending-timer-label m))
+                           (heap-remove-min! heap)
+                           (hash-remove! timers label)
+                           (log-syndicate/drivers/timer-debug "expired timer ~a" label)
+                           (ground-send! (timer-expired label now))
+                           (set! count-fired (+ count-fired 1))
+                           (loop)))))
                    (signal-background-activity! (- count-fired)))
 
+                 (define (clear-timer! label)
+                   (match (hash-ref timers label #f)
+                     [#f (void)]
+                     [deadline
+                      (heap-remove! heap (pending-timer deadline label))
+                      (hash-remove! timers label)
+                      (signal-background-activity! -1)]))
+
                  (define (install-timer! label deadline)
+                   (clear-timer! label)
                    (heap-add! heap (pending-timer deadline label))
+                   (hash-set! timers label deadline)
                    (signal-background-activity! +1))
 
                  (let loop ()
@@ -64,6 +77,9 @@
                                             (loop)))])
                          (handle-evt control-ch
                                      (match-lambda
+                                       [(set-timer label _ 'clear)
+                                        (clear-timer! label)
+                                        (loop)]
                                        [(set-timer label msecs 'relative)
                                         (define deadline (+ (current-inexact-milliseconds) msecs))
                                         (install-timer! label deadline)
@@ -82,9 +98,7 @@
                                             (current-inexact-milliseconds))
          (define timer-id (gensym 'timestate))
          (on-start (send! (set-timer timer-id msecs 'absolute)))
-         ;; TODO: on-stop to delete irrelevant timers, so that their background-activity is
-         ;; removed, so that programs don't stay running uselessly while previously-set but
-         ;; now-unwanted timers expire unobserved.
+         (on-stop (send! (set-timer timer-id msecs 'clear)))
          (on (message (timer-expired timer-id _))
              (react (assert (later-than msecs))))))