diff --git a/core/divelist.cpp b/core/divelist.cpp
index d17a3bce2..4dfa8d022 100644
--- a/core/divelist.cpp
+++ b/core/divelist.cpp
@@ -1270,17 +1270,15 @@ bool dive_or_trip_less_than(struct dive_or_trip a, struct dive_or_trip b)
  * that happened inside other dives. The interval will always be calculated
  * with respect to the dive that started previously.
  */
-timestamp_t get_surface_interval(timestamp_t when)
+timestamp_t dive_table::get_surface_interval(timestamp_t when) const
 {
-	timestamp_t prev_end;
-
 	/* find previous dive. might want to use a binary search. */
-	auto it = std::find_if(divelog.dives.rbegin(), divelog.dives.rend(),
+	auto it = std::find_if(rbegin(), rend(),
 			       [when] (auto &d) { return d->when < when; });
-	if (it == divelog.dives.rend())
+	if (it == rend())
 		return -1;
 
-	prev_end = (*it)->endtime();
+	timestamp_t prev_end = (*it)->endtime();
 	if (prev_end > when)
 		return 0;
 	return when - prev_end;
diff --git a/core/divelist.h b/core/divelist.h
index 946d87638..2942c9da1 100644
--- a/core/divelist.h
+++ b/core/divelist.h
@@ -21,6 +21,7 @@ struct dive_table : public sorted_owning_table<dive, &comp_dives> {
 	void record_dive(std::unique_ptr<dive> d);	// call fixup_dive() before adding dive to table.
 	std::unique_ptr<dive> unregister_dive(int idx);
 
+	timestamp_t get_surface_interval(timestamp_t when) const;
 	struct dive *find_next_visible_dive(timestamp_t when);
 };
 
@@ -51,7 +52,6 @@ extern process_imported_dives_result process_imported_dives(struct divelog &impo
 
 extern void get_dive_gas(const struct dive *dive, int *o2_p, int *he_p, int *o2low_p);
 extern int get_dive_nr_at_idx(int idx);
-extern timestamp_t get_surface_interval(timestamp_t when);
 
 int get_min_datafile_version();
 void report_datafile_version(int version);
diff --git a/desktop-widgets/tab-widgets/TabDiveInformation.cpp b/desktop-widgets/tab-widgets/TabDiveInformation.cpp
index f08594fe0..a0c430205 100644
--- a/desktop-widgets/tab-widgets/TabDiveInformation.cpp
+++ b/desktop-widgets/tab-widgets/TabDiveInformation.cpp
@@ -167,7 +167,7 @@ void TabDiveInformation::updateProfile()
 // Update fields that depend on start of dive
 void TabDiveInformation::updateWhen()
 {
-	timestamp_t surface_interval = get_surface_interval(parent.currentDive->when);
+	timestamp_t surface_interval = divelog.dives.get_surface_interval(parent.currentDive->when);
 	if (surface_interval >= 0)
 		ui->surfaceIntervalText->setText(get_dive_surfint_string(surface_interval, tr("d"), tr("h"), tr("min")));
 	else