diff --git a/src/core/metrics.ml b/src/core/metrics.ml
index dcf9efa3..988b7bd6 100644
--- a/src/core/metrics.ml
+++ b/src/core/metrics.ml
@@ -20,19 +20,17 @@ open struct
 end
 
 (** Number data point, as a float *)
-let float ?(start_time_unix_nano = _program_start)
-    ?(now = Timestamp_ns.now_unix_ns ()) ?(attrs = []) (d : float) :
-    number_data_point =
+let float ?start_time_unix_nano ?(now = Timestamp_ns.now_unix_ns ())
+    ?(attrs = []) (d : float) : number_data_point =
   let attributes = attrs |> List.map Key_value.conv in
-  make_number_data_point ~start_time_unix_nano ~time_unix_nano:now ~attributes
+  make_number_data_point ?start_time_unix_nano ~time_unix_nano:now ~attributes
     ~value:(As_double d) ()
 
 (** Number data point, as an int *)
-let int ?(start_time_unix_nano = _program_start)
-    ?(now = Timestamp_ns.now_unix_ns ()) ?(attrs = []) (i : int) :
-    number_data_point =
+let int ?start_time_unix_nano ?(now = Timestamp_ns.now_unix_ns ()) ?(attrs = [])
+    (i : int) : number_data_point =
   let attributes = attrs |> List.map Key_value.conv in
-  make_number_data_point ~start_time_unix_nano ~time_unix_nano:now ~attributes
+  make_number_data_point ?start_time_unix_nano ~time_unix_nano:now ~attributes
     ~value:(As_int (Int64.of_int i))
     ()
 
@@ -62,12 +60,12 @@ let sum ~name ?description ?unit_
       count value of histogram for each bucket. Sum of the counts must be equal
       to [count]. length must be [1+length explicit_bounds]
     @param explicit_bounds strictly increasing list of bounds for the buckets *)
-let histogram_data_point ?(start_time_unix_nano = _program_start)
+let histogram_data_point ?start_time_unix_nano
     ?(now = Timestamp_ns.now_unix_ns ()) ?(attrs = []) ?(exemplars = [])
     ?(explicit_bounds = []) ?sum ~bucket_counts ~count () : histogram_data_point
     =
   let attributes = attrs |> List.map Key_value.conv in
-  make_histogram_data_point ~start_time_unix_nano ~time_unix_nano:now
+  make_histogram_data_point ?start_time_unix_nano ~time_unix_nano:now
     ~attributes ~exemplars ~bucket_counts ~explicit_bounds ~count ?sum ()
 
 let histogram ~name ?description ?unit_ ?aggregation_temporality