diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/dune b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/dune
index d8036380cd..5d61f52cfc 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/dune
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/dune
@@ -18,7 +18,7 @@
   (public_name xapi-stdext-threads.scheduler)
   (name xapi_stdext_threads_scheduler)
   (modules ipq scheduler)
-  (libraries mtime mtime.clock.os threads.posix unix xapi-log xapi-stdext-threads)
+  (libraries mtime mtime.clock.os threads.posix unix xapi-log xapi-stdext-threads clock)
 )
 
 (tests
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq.ml
index 4cf29ed3d9..7293ae625e 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq.ml
@@ -13,7 +13,7 @@
  *)
 (* Imperative priority queue *)
 
-type 'a event = {ev: 'a; time: Mtime.t}
+type 'a event = {ev: 'a; time: Mtime.span}
 
 type 'a t = {default: 'a event; mutable size: int; mutable data: 'a event array}
 
@@ -23,7 +23,7 @@ let create n default =
   if n <= 0 then
     invalid_arg "create"
   else
-    let default = {ev= default; time= Mtime_clock.now ()} in
+    let default = {ev= default; time= Mtime_clock.elapsed ()} in
     {default; size= 0; data= Array.make n default}
 
 let is_empty h = h.size <= 0
@@ -45,7 +45,7 @@ let add h x =
   (* moving [x] up in the heap *)
   let rec moveup i =
     let fi = (i - 1) / 2 in
-    if i > 0 && Mtime.is_later d.(fi).time ~than:x.time then (
+    if i > 0 && Mtime.Span.is_longer d.(fi).time ~than:x.time then (
       d.(i) <- d.(fi) ;
       moveup fi
     ) else
@@ -69,7 +69,7 @@ let remove h s =
   (* moving [x] up in the heap *)
   let rec moveup i =
     let fi = (i - 1) / 2 in
-    if i > 0 && Mtime.is_later d.(fi).time ~than:x.time then (
+    if i > 0 && Mtime.Span.is_longer d.(fi).time ~than:x.time then (
       d.(i) <- d.(fi) ;
       moveup fi
     ) else
@@ -83,7 +83,7 @@ let remove h s =
         let j' = j + 1 in
         if j' < n && d.(j').time < d.(j).time then j' else j
       in
-      if Mtime.is_earlier d.(j).time ~than:x.time then (
+      if Mtime.Span.is_shorter d.(j).time ~than:x.time then (
         d.(i) <- d.(j) ;
         movedown j
       ) else
@@ -93,7 +93,7 @@ let remove h s =
   in
   if s = n then
     ()
-  else if Mtime.is_later d.(s).time ~than:x.time then
+  else if Mtime.Span.is_longer d.(s).time ~than:x.time then
     moveup s
   else
     movedown s ;
@@ -129,7 +129,7 @@ let check h =
   let d = h.data in
   for i = 1 to h.size - 1 do
     let fi = (i - 1) / 2 in
-    let ordered = Mtime.is_later d.(i).time ~than:d.(fi).time in
+    let ordered = Mtime.Span.is_longer d.(i).time ~than:d.(fi).time in
     assert ordered
   done
 
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq.mli b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq.mli
index b7c4974e64..19f8bf1e33 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq.mli
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq.mli
@@ -12,7 +12,7 @@
  * GNU Lesser General Public License for more details.
  *)
 
-type 'a event = {ev: 'a; time: Mtime.t}
+type 'a event = {ev: 'a; time: Mtime.span}
 
 type 'a t
 
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq_test.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq_test.ml
index e8e64093e1..a9cc2611da 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq_test.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/ipq_test.ml
@@ -17,7 +17,7 @@ module Ipq = Xapi_stdext_threads_scheduler.Ipq
 (* test we get "out of bound" exception calling Ipq.remove *)
 let test_out_of_index () =
   let q = Ipq.create 10 0 in
-  Ipq.add q {Ipq.ev= 123; Ipq.time= Mtime_clock.now ()} ;
+  Ipq.add q {Ipq.ev= 123; Ipq.time= Mtime_clock.elapsed ()} ;
   let is_oob = function
     | Invalid_argument s when String.ends_with ~suffix:"  out of bounds" s ->
         true
@@ -43,18 +43,18 @@ let test_leak () =
   let use_array () = array.(0) <- 'a' in
   let allocated = Atomic.make true in
   Gc.finalise (fun _ -> Atomic.set allocated false) array ;
-  Ipq.add q {Ipq.ev= use_array; Ipq.time= Mtime_clock.now ()} ;
+  Ipq.add q {Ipq.ev= use_array; Ipq.time= Mtime_clock.elapsed ()} ;
   Ipq.remove q 0 ;
   Gc.full_major () ;
   Gc.full_major () ;
   Alcotest.(check bool) "allocated" false (Atomic.get allocated) ;
-  Ipq.add q {Ipq.ev= default; Ipq.time= Mtime_clock.now ()}
+  Ipq.add q {Ipq.ev= default; Ipq.time= Mtime_clock.elapsed ()}
 
 (* test Ipq.is_empty call *)
 let test_empty () =
   let q = Ipq.create 10 0 in
   Alcotest.(check bool) "same value" true (Ipq.is_empty q) ;
-  Ipq.add q {Ipq.ev= 123; Ipq.time= Mtime_clock.now ()} ;
+  Ipq.add q {Ipq.ev= 123; Ipq.time= Mtime_clock.elapsed ()} ;
   Alcotest.(check bool) "same value" false (Ipq.is_empty q) ;
   Ipq.remove q 0 ;
   Alcotest.(check bool) "same value" true (Ipq.is_empty q)
@@ -75,7 +75,7 @@ let set queue =
   Ipq.iter
     (fun d ->
       let t = d.time in
-      let t = Mtime.to_uint64_ns t in
+      let t = Mtime.Span.to_uint64_ns t in
       s := Int64Set.add t !s
     )
     queue ;
@@ -86,7 +86,7 @@ let test_old () =
   let s = ref Int64Set.empty in
   let add i =
     let ti = Random.int64 1000000L in
-    let t = Mtime.of_uint64_ns ti in
+    let t = Mtime.Span.of_uint64_ns ti in
     let e = {Ipq.time= t; Ipq.ev= i} in
     Ipq.add test e ;
     s := Int64Set.add ti !s
@@ -123,7 +123,7 @@ let test_old () =
   let prev = ref 0L in
   for _ = 0 to 49 do
     let e = Ipq.pop_maximum test in
-    let t = Mtime.to_uint64_ns e.time in
+    let t = Mtime.Span.to_uint64_ns e.time in
     Alcotest.(check bool)
       (Printf.sprintf "%Ld bigger than %Ld" t !prev)
       true (t >= !prev) ;
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler.ml
index a544ed79bb..27cf306995 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler.ml
@@ -33,31 +33,23 @@ let (queue : t Ipq.t) = Ipq.create 50 queue_default
 
 let lock = Mutex.create ()
 
-module Clock = struct
-  let span s = Mtime.Span.of_uint64_ns (Int64.of_float (s *. 1e9))
-
-  let span_to_s span =
-    Mtime.Span.to_uint64_ns span |> Int64.to_float |> fun ns -> ns /. 1e9
-
-  let add_span clock secs =
-    (* return mix or max available value if the add overflows *)
-    match Mtime.add_span clock (span secs) with
-    | Some t ->
-        t
-    | None when secs > 0. ->
-        Mtime.max_stamp
-    | None ->
-        Mtime.min_stamp
-end
-
-let add_to_queue name ty start newfunc =
-  let ( ++ ) = Clock.add_span in
+let add_to_queue_span name ty start_span newfunc =
+  let ( ++ ) = Mtime.Span.add in
   let item =
-    {Ipq.ev= {func= newfunc; ty; name}; Ipq.time= Mtime_clock.now () ++ start}
+    {
+      Ipq.ev= {func= newfunc; ty; name}
+    ; Ipq.time= Mtime_clock.elapsed () ++ start_span
+    }
   in
   with_lock lock (fun () -> Ipq.add queue item) ;
   Delay.signal delay
 
+let add_to_queue name ty start newfunc =
+  let start_span =
+    Clock.Timer.s_to_span start |> Option.value ~default:Mtime.Span.max_span
+  in
+  add_to_queue_span name ty start_span newfunc
+
 let remove_from_queue name =
   with_lock lock @@ fun () ->
   match !pending_event with
@@ -72,8 +64,11 @@ let add_periodic_pending () =
   with_lock lock @@ fun () ->
   match !pending_event with
   | Some ({ty= Periodic timer; _} as ev) ->
-      let ( ++ ) = Clock.add_span in
-      let item = {Ipq.ev; Ipq.time= Mtime_clock.now () ++ timer} in
+      let ( ++ ) = Mtime.Span.add in
+      let delta =
+        Clock.Timer.s_to_span timer |> Option.value ~default:Mtime.Span.max_span
+      in
+      let item = {Ipq.ev; Ipq.time= Mtime_clock.elapsed () ++ delta} in
       Ipq.add queue item ;
       pending_event := None
   | Some {ty= OneShot; _} ->
@@ -85,15 +80,15 @@ let loop () =
   debug "%s started" __MODULE__ ;
   try
     while true do
-      let now = Mtime_clock.now () in
+      let now = Mtime_clock.elapsed () in
       let deadline, item =
         with_lock lock @@ fun () ->
         (* empty: wait till we get something *)
         if Ipq.is_empty queue then
-          (Clock.add_span now 10.0, None)
+          (Mtime.Span.add now Mtime.Span.(10 * s), None)
         else
           let next = Ipq.maximum queue in
-          if Mtime.is_later next.Ipq.time ~than:now then
+          if Mtime.Span.is_longer next.Ipq.time ~than:now then
             (* not expired: wait till time or interrupted *)
             (next.Ipq.time, None)
           else (
@@ -111,7 +106,9 @@ let loop () =
       | None -> (
           (* Sleep until next event. *)
           let sleep =
-            Mtime.(span deadline now) |> Mtime.Span.(add ms) |> Clock.span_to_s
+            Mtime.(Span.abs_diff deadline now)
+            |> Mtime.Span.(add ms)
+            |> Clock.Timer.span_to_s
           in
           try ignore (Delay.wait delay sleep)
           with e ->
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler.mli b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler.mli
index d4d19b1f79..53a7c764dc 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler.mli
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler.mli
@@ -18,6 +18,10 @@ type func_ty =
   | OneShot  (** Fire just once *)
   | Periodic of float  (** Fire periodically with a given period in seconds *)
 
+val add_to_queue_span :
+  string -> func_ty -> Mtime.span -> (unit -> unit) -> unit
+(** Start a new timer. *)
+
 val add_to_queue : string -> func_ty -> float -> (unit -> unit) -> unit
 (** Start a new timer. *)
 
diff --git a/ocaml/xapi/xapi_event.ml b/ocaml/xapi/xapi_event.ml
index af2a610523..94b7c4bd9b 100644
--- a/ocaml/xapi/xapi_event.ml
+++ b/ocaml/xapi/xapi_event.ml
@@ -419,20 +419,25 @@ module From = struct
 
   let session_is_invalid call = with_lock call.m (fun () -> call.session_invalid)
 
-  let wait2 call from_id deadline =
+  let wait2 call from_id timer =
     let timeoutname = Printf.sprintf "event_from_timeout_%Ld" call.index in
     with_lock m (fun () ->
         while
           from_id = call.cur_id
           && (not (session_is_invalid call))
-          && Unix.gettimeofday () < deadline
+          && not (Clock.Timer.has_expired timer)
         do
-          Xapi_stdext_threads_scheduler.Scheduler.add_to_queue timeoutname
-            Xapi_stdext_threads_scheduler.Scheduler.OneShot
-            (deadline -. Unix.gettimeofday () +. 0.5)
-            (fun () -> Condition.broadcast c) ;
-          Condition.wait c m ;
-          Xapi_stdext_threads_scheduler.Scheduler.remove_from_queue timeoutname
+          match Clock.Timer.remaining timer with
+          | Expired _ ->
+              ()
+          | Remaining delta ->
+              Xapi_stdext_threads_scheduler.Scheduler.add_to_queue_span
+                timeoutname Xapi_stdext_threads_scheduler.Scheduler.OneShot
+                delta (fun () -> Condition.broadcast c
+              ) ;
+              Condition.wait c m ;
+              Xapi_stdext_threads_scheduler.Scheduler.remove_from_queue
+                timeoutname
         done
     ) ;
     if session_is_invalid call then (
@@ -506,7 +511,7 @@ let rec next ~__context =
   else
     rpc_of_events relevant
 
-let from_inner __context session subs from from_t deadline =
+let from_inner __context session subs from from_t timer =
   let open Xapi_database in
   let open From in
   (* The database tables involved in our subscription *)
@@ -605,14 +610,14 @@ let from_inner __context session subs from from_t deadline =
             && mods = []
             && deletes = []
             && messages = []
-            && Unix.gettimeofday () < deadline
+            && not (Clock.Timer.has_expired timer)
           then (
             last_generation := last ;
             (* Cur_id was bumped, but nothing relevent fell out of the db. Therefore the *)
             sub.cur_id <- last ;
             (* last id the client got is equivalent to the current one *)
             last_msg_gen := msg_gen ;
-            wait2 sub last deadline ;
+            wait2 sub last timer ;
             Thread.delay 0.05 ;
             grab_nonempty_range ()
           ) else
@@ -705,14 +710,19 @@ let from ~__context ~classes ~token ~timeout =
         )
   in
   let subs = List.map Subscription.of_string classes in
-  let deadline = Unix.gettimeofday () +. timeout in
+  let duration =
+    timeout
+    |> Clock.Timer.s_to_span
+    |> Option.value ~default:Mtime.Span.(24 * hour)
+  in
+  let timer = Clock.Timer.start ~duration in
   (* We need to iterate because it's possible for an empty event set
      	   to be generated if we peek in-between a Modify and a Delete; we'll
      	   miss the Delete event and fail to generate the Modify because the
      	   snapshot can't be taken. *)
   let rec loop () =
-    let event_from = from_inner __context session subs from from_t deadline in
-    if event_from.events = [] && Unix.gettimeofday () < deadline then (
+    let event_from = from_inner __context session subs from from_t timer in
+    if event_from.events = [] && not (Clock.Timer.has_expired timer) then (
       debug "suppressing empty event.from" ;
       loop ()
     ) else