diff --git a/_02_.Writing-programs.sz b/_02_.Writing-programs.sz
index ffb9fff..39bce26 100644
--- a/_02_.Writing-programs.sz
+++ b/_02_.Writing-programs.sz
@@ -955,7 +955,7 @@ Functions that may suspend execution—whether waiting on a channel, sleeping, o
 @codeblock[#:lang "bjolang"]|{
 (: think (-bjo-> int int))
 (defbjo (think n)
-  (sync (timeout 50))     ;; wait 50 ms, without holding up a thread
+  (sync (timeout (milliseconds 50)))     ;; wait 50 ms, without holding up a thread
   (* n n))
 }|
 
@@ -1035,29 +1035,31 @@ Crucially, calling @code{(chan-recv seat)} does not perform the receive immediat
 |---------------------------+-------------------------------------------------------------|
 | @code{(chan-recv ch)}     | Receives a value from channel @code{ch}                     |
 | @code{(chan-send ch v)}   | Sends @code{v} on channel @code{ch}                         |
-| @code{(timeout ms)}       | Fires after @code{ms} milliseconds                          |
+| @code{(timeout d)}        | Fires after the @code{Duration} @code{d}                    |
 | @code{(choose e1 e2 ...)} | Fires when the first of the supplied events occurs          |
 | @code{(wrap e f)}         | Fires when @code{e} occurs, applying @code{f} to its value  |
 }
 
+A @code{Duration} is a length of time, made with @code{milliseconds}, @code{seconds}, @code{minutes} or @code{hours}: @code{(timeout (seconds 5))}. It has its unit in its type, so a timer cannot be given a bare number, and one too long for its range raises rather than wrapping round.
+
 With these combinators, deadlines and timeouts become simple functions that compose over any event. For instance, we can enforce a response timeout on our card players:
 
 @codeblock[#:lang "bjolang"]|{
-(: within (-> int (Event %a) (Event (Option %a))))
-(defun (within ms ev)
+(: within (-> Duration (Event %a) (Event (Option %a))))
+(defun (within d ev)
   (choose (wrap ev #(Some &))
-          (wrap (timeout ms) (fun (u) None))))
+          (wrap (timeout d) (fun (u) None))))
 
 (: slow-player (-bjo-> (Chan string) void))
 (defbjo (slow-player seat)
-  (sync (timeout 200))
+  (sync (timeout (milliseconds 200)))
   (sync (chan-send seat "7♣")))
 
 (defbjo (main)
   (def seat (make-chan))
   (spawn (slow-player seat))
-  (println (sync (within 50 (chan-recv seat))))    ;; None
-  (println (sync (within 500 (chan-recv seat))))   ;; (Some 7♣)
+  (println (sync (within (milliseconds 50) (chan-recv seat))))    ;; None
+  (println (sync (within (milliseconds 500) (chan-recv seat))))   ;; (Some 7♣)
   0)
 }|
 
@@ -1097,14 +1099,14 @@ The top-level @code{main} function acts as an implicit root scope, which is why
 |---------------------------------------+------------------------------------------------------------|
 | @code{(with-scope body ...)}          | Waits for child fibers and finalizes open file ports       |
 | @code{(with-cancel (cancel) body ...)} | Binds a @code{cancel} procedure to signal child fibers     |
-| @code{(with-deadline ms body ...)}    | Cancels the scope automatically after @code{ms} ms         |
+| @code{(with-deadline d body ...)}     | Cancels the scope automatically after the duration @code{d} |
 | @code{(with-shield body ...)}         | Shields cleanup operations from parent cancellation       |
 }
 
 @codeblock[#:lang "bjolang"]|{
 (: fetch (-bjo-> string void))
 (defbjo (fetch name)
-  (sync (timeout 30))
+  (sync (timeout (milliseconds 30)))
   (println #"fetched ${name}"))
 
 (: count-up (-bjo-> (Chan int) void))
@@ -1115,7 +1117,7 @@ The top-level @code{main} function acts as an implicit root scope, which is why
 
 (defbjo (main)
   ;; Both are fetched when this returns, or the deadline raises.
-  (with-deadline 1000
+  (with-deadline (seconds 1)
     (spawn (fetch "a"))
     (spawn (fetch "b")))
   (println "both done")
@@ -1249,7 +1251,7 @@ If a transaction raises an exception, it writes nothing, and @code{atomically} r
 @codeblock[#:lang "bjolang"]|{
 ;; A card from the pile, or None after 100 ms.
 (sync (choose (wrap (atomically-evt (fun (tx) (take-top tx pile))) Some)
-              (wrap (timeout 100) (fun (_) None))))
+              (wrap (timeout (milliseconds 100)) (fun (_) None))))
 }|
 
 Inside a transaction, @code{(tx-or-else tx a b)} runs @code{a}, and if @code{a} retries, undoes its writes and runs @code{b}. If both retry, the whole transaction waits for a change to anything either of them read.