diff --git a/lwt/prometheus_lwt.ml b/lwt/prometheus_lwt.ml
index d0cd169..1744766 100644
--- a/lwt/prometheus_lwt.ml
+++ b/lwt/prometheus_lwt.ml
@@ -22,13 +22,13 @@ end
 module Gauge = struct
   let track_in_progress = Prometheus.Gauge.track_inprogress
 
-  let set_time = Prometheus.Gauge.time
+  let set_time ~gettime t fn = Prometheus.Gauge.time t gettime fn
 end
 
 module Summary = struct
-  let observe_time = Prometheus.Summary.time
+  let observe_time ~gettime t fn = Prometheus.Summary.time t gettime fn
 end
 
 module Histogram (H : Prometheus.HISTOGRAM) = struct
-  let observe_time = H.time
+  let observe_time ~gettime t fn = H.time t gettime fn
 end
diff --git a/lwt/prometheus_lwt.mli b/lwt/prometheus_lwt.mli
index 355c981..7bb59a7 100644
--- a/lwt/prometheus_lwt.mli
+++ b/lwt/prometheus_lwt.mli
@@ -33,17 +33,17 @@ module Gauge : sig
   val track_in_progress : Prometheus.Gauge.t -> (unit -> 'a Lwt.t) -> 'a Lwt.t
   (** [track_in_progress t f] increases [t] by one while [f ()] is running. *)
 
-  val set_time : Prometheus.Gauge.t -> (unit -> float) -> (unit -> 'a Lwt.t) -> 'a Lwt.t
-  (** [set_time t gettime f] calls [gettime ()] before and after executing [f ()] and
+  val set_time : gettime:(unit -> float) -> Prometheus.Gauge.t -> (unit -> 'a Lwt.t) -> 'a Lwt.t
+  (** [set_time ~gettime t f] calls [gettime ()] before and after executing [f ()] and
       sets [t] to the difference. *)
 end
 
 module Summary : sig
-  val observe_time : Prometheus.Summary.t -> (unit -> float) -> (unit -> 'a Lwt.t) -> 'a Lwt.t
-  (** [observe_time t gettime f] observes the duration of [f ()]. *)
+  val observe_time : gettime:(unit -> float) -> Prometheus.Summary.t -> (unit -> 'a Lwt.t) -> 'a Lwt.t
+  (** [observe_time ~gettime t f] observes the duration of [f ()]. *)
 end
 
 module Histogram (H : Prometheus.HISTOGRAM) : sig
-  val observe_time : H.t -> (unit -> float) -> (unit -> 'a Lwt.t) -> 'a Lwt.t
-  (** [observe_time t gettime f] observes the duration of [f ()]. *)
+  val observe_time : gettime:(unit -> float) -> H.t -> (unit -> 'a Lwt.t) -> 'a Lwt.t
+  (** [observe_time ~gettime t f] observes the duration of [f ()]. *)
 end
diff --git a/lwt/test/test_lwt.ml b/lwt/test/test_lwt.ml
index 328937f..6d768fa 100644
--- a/lwt/test/test_lwt.ml
+++ b/lwt/test/test_lwt.ml
@@ -46,7 +46,7 @@ let test_timers () =
   let gauge = Gauge.v ~registry:core ~help:"Time taken" "gauge_time" in
   let clock = ref 0.0 in
   let gettime () = !clock in
-  Prometheus_lwt.Gauge.set_time gauge gettime
+  Prometheus_lwt.Gauge.set_time gauge ~gettime
     (fun () -> clock := !clock +. 1.5; Lwt.pause ())
   >>= fun () ->
   Prometheus_lwt.CollectorRegistry.collect registry >|= fun collected ->
diff --git a/src/prometheus.ml b/src/prometheus.ml
index 4b28582..e960aec 100644
--- a/src/prometheus.ml
+++ b/src/prometheus.ml
@@ -266,11 +266,11 @@ module Gauge = struct
          Lwt.return_unit
       )
 
-  let set_time t gettimeofday fn =
-    let start = gettimeofday () in
+  let set_time ~gettime t fn =
+    let start = gettime () in
     Fun.protect fn
       ~finally:(fun () ->
-         let finish = gettimeofday () in
+         let finish = gettime () in
          set t (finish -. start)
       )
 end
@@ -309,11 +309,11 @@ module Summary = struct
          Lwt.return_unit
       )
 
-  let observe_time t gettimeofday fn =
-    let start = gettimeofday () in
+  let observe_time ~gettime t fn =
+    let start = gettime () in
     Fun.protect fn
       ~finally:(fun () ->
-         let finish = gettimeofday () in
+         let finish = gettime () in
          observe t (finish -. start)
       )
 end
@@ -365,7 +365,7 @@ module type HISTOGRAM = sig
   include METRIC
   val observe : t -> float -> unit
   val time : t -> (unit -> float) -> (unit -> 'a Lwt.t) -> 'a Lwt.t [@@deprecated]
-  val observe_time : t -> (unit -> float) -> (unit -> 'a) -> 'a
+  val observe_time : gettime:(unit -> float) -> t -> (unit -> 'a) -> 'a
 end
 
 let bucket_label = LabelName.v "le"
@@ -422,11 +422,11 @@ module Histogram (Buckets : BUCKETS) = struct
          Lwt.return_unit
       )
 
-  let observe_time t gettimeofday fn =
-    let start = gettimeofday () in
+  let observe_time ~gettime t fn =
+    let start = gettime () in
     Fun.protect fn
       ~finally:(fun () ->
-         let finish = gettimeofday () in
+         let finish = gettime () in
          observe t (finish -. start)
       )
 end
diff --git a/src/prometheus.mli b/src/prometheus.mli
index 14e1aa1..5836cdb 100644
--- a/src/prometheus.mli
+++ b/src/prometheus.mli
@@ -180,8 +180,8 @@ module Gauge : sig
   (** [time t gettime f] calls [gettime ()] before and after executing [f ()] and
       increases the metric by the difference. *)
 
-  val set_time : t -> (unit -> float) -> (unit -> 'a) -> 'a
-  (** [set_time t gettime f] calls [gettime ()] before and after executing [f ()] and
+  val set_time : gettime:(unit -> float) -> t -> (unit -> 'a) -> 'a
+  (** [set_time ~gettime t f] calls [gettime ()] before and after executing [f ()] and
       sets [t] to the difference. *)
 end
 (** A gauge is a metric that represents a single numerical value that can arbitrarily go up and down. *)
@@ -197,8 +197,8 @@ module Summary : sig
   (** [time t gettime f] calls [gettime ()] before and after executing [f ()] and
       observes the difference. *)
 
-  val observe_time : t -> (unit -> float) -> (unit -> 'a) -> 'a
-  (** [observe_time t gettime f] calls [gettime ()] before and after executing [f ()] and
+  val observe_time : gettime:(unit -> float) -> t -> (unit -> 'a) -> 'a
+  (** [observe_time ~gettime t f] calls [gettime ()] before and after executing [f ()] and
       observes the difference. *)
 end
 (** A summary is a metric that records both the number of readings and their total.
@@ -235,8 +235,8 @@ module type HISTOGRAM = sig
   (** [time t gettime f] calls [gettime ()] before and after executing [f ()] and
       observes the difference. *)
 
-  val observe_time : t -> (unit -> float) -> (unit -> 'a) -> 'a
-  (** [observe_time t gettime f] calls [gettime ()] before and after executing [f ()] and
+  val observe_time : gettime:(unit -> float) -> t -> (unit -> 'a) -> 'a
+  (** [observe_time ~gettime t f] calls [gettime ()] before and after executing [f ()] and
       observes the difference. *)
 end
 
diff --git a/tests/test.ml b/tests/test.ml
index 9848dec..8f08250 100644
--- a/tests/test.ml
+++ b/tests/test.ml
@@ -99,11 +99,11 @@ let test_sync_timers () =
   let summary = Summary.v ~registry ~help:"Time taken" "summary_time" in
   let clock = ref 0.0 in
   let gettime () = !clock in
-  Gauge.set_time gauge gettime (fun () -> clock := !clock +. 1.3);
-  Gauge.set_time gauge gettime (fun () -> clock := !clock +. 1.5);
+  Gauge.set_time gauge ~gettime (fun () -> clock := !clock +. 1.3);
+  Gauge.set_time gauge ~gettime (fun () -> clock := !clock +. 1.5);
   Gauge.track_in_progress gauge (fun () -> ());
-  Summary.observe_time summary gettime (fun () -> clock := !clock +. 0.5);
-  Summary.observe_time summary gettime (fun () -> clock := !clock +. 1.5);
+  Summary.observe_time summary ~gettime (fun () -> clock := !clock +. 0.5);
+  Summary.observe_time summary ~gettime (fun () -> clock := !clock +. 1.5);
   CollectorRegistry.collect registry >|= fun collected ->
   let output = Fmt.to_to_string TextFormat_0_0_4.output collected in
   Alcotest.(check string) "Text output"