diff --git a/etc/db/upgrades/upgrade22-v0.3.9-add-replace-placeholders-function.sql b/etc/db/upgrades/upgrade22-v0.3.9-add-replace-placeholders-function.sql
new file mode 100644
index 0000000..c0a0be4
--- /dev/null
+++ b/etc/db/upgrades/upgrade22-v0.3.9-add-replace-placeholders-function.sql
@@ -0,0 +1,229 @@
+-- Fix not being able to edit a time-based event.
+--
+-- New schema version: 0.3.9
+--
+-- ATTENTION:
+--
+-- ENSURE YOU HAVE BACKED UP THE DATABASE BEFORE RUNNING THIS SCRIPT.
+--
+--
+-- NOTE: This upgrade affects all schemas in the database.
+-- NOTE: Each application starts a transaction, which must be committed
+-- or rolled back.
+--
+-- This defines a replace_placeholders() function, taking as arguments
+-- a text string and either a timestamp or a sequence / point pair. It
+-- uses the latter arguments to find metadata from which it can extract
+-- relevant information and replace it into the text string wherever the
+-- appropriate placeholders appear. For instance, given a call such as
+-- replace_placeholders('The position is @POS@', NULL, 11, 2600) it will
+-- replace '@POS@' with the position of point 2600 in sequence 11, if it
+-- exists (or leave the placeholder untouched otherwise).
+--
+-- A scan_placeholders() procedure is also defined, which calls the above
+-- function on the entire event log.
+--
+-- To apply, run as the dougal user:
+--
+-- psql <<EOF
+-- \i $THIS_FILE
+-- COMMIT;
+-- EOF
+--
+-- NOTE: It can be applied multiple times without ill effect.
+--
+
+BEGIN;
+
+CREATE OR REPLACE PROCEDURE show_notice (notice text) AS $$
+BEGIN
+	RAISE NOTICE '%', notice;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE PROCEDURE pg_temp.upgrade_survey_schema (schema_name text) AS $outer$
+BEGIN
+
+	RAISE NOTICE 'Updating schema %', schema_name;
+	-- We need to set the search path because some of the trigger
+	-- functions reference other tables in survey schemas assuming
+	-- they are in the search path.
+	EXECUTE format('SET search_path TO %I,public', schema_name);
+
+	CREATE OR REPLACE FUNCTION replace_placeholders (
+		text_in text, tstamp timestamptz, sequence integer, point integer
+	)
+	RETURNS text
+	AS $$
+	DECLARE
+		position geometry;
+		metadata jsonb;
+		text_out text;
+
+		json_query text;
+		json_result jsonb;
+		expect_recursion boolean := false;
+	BEGIN
+
+		text_out := text_in;
+
+		-- We only get a position if we are going to need it…
+		IF regexp_match(text_out, '@DMS@|@POS@|@DEG@') IS NOT NULL THEN
+			position := ST_Transform(event_position(tstamp, sequence, point), 4326);
+		END IF;
+
+		-- …and likewise with the metadata.
+		IF regexp_match(text_out, '@BSP@|@WD@|@CMG@|@EN@|@GRID@|@(\$\..*?)@@') IS NOT NULL THEN
+			metadata := event_meta(tstamp, sequence, point);
+		END IF;
+
+		-- We shortcut the evaluation if neither of the above regexps matched
+		IF position IS NULL AND metadata IS NULL THEN
+			RETURN text_out;
+		END IF;
+
+		IF position('@DMS@' IN text_out) != 0 THEN
+			text_out := replace(text_out, '@DMS@', ST_AsLatLonText(position));
+		END IF;
+
+		IF position('@POS@' IN text_out) != 0 THEN
+			text_out := replace(text_out, '@POS@', replace(ST_AsLatLonText(position, 'D.DDDDDD'), ' ', ', '));
+		END IF;
+
+		IF position('@DEG@' IN text_out) != 0 THEN
+			text_out := replace(text_out, '@DEG@', replace(ST_AsLatLonText(position, 'D.DDDDDD'), ' ', ', '));
+		END IF;
+
+		IF position('@EN@' IN text_out) != 0 THEN
+			IF metadata ? 'easting' AND metadata ? 'northing' THEN
+				text_out := replace(text_out, '@EN@', (metadata->>'easting') || ', ' || (metadata->>'northing'));
+			END IF;
+		END IF;
+
+		IF position('@GRID@' IN text_out) != 0 THEN
+			IF metadata ? 'easting' AND metadata ? 'northing' THEN
+				text_out := replace(text_out, '@GRID@', (metadata->>'easting') || ', ' || (metadata->>'northing'));
+			END IF;
+		END IF;
+
+		IF position('@CMG@' IN text_out) != 0 THEN
+			IF metadata ? 'bearing' THEN
+				text_out := replace(text_out, '@CMG@', metadata->>'bearing');
+			END IF;
+		END IF;
+
+		IF position('@BSP@' IN text_out) != 0 THEN
+			IF metadata ? 'speed' THEN
+				text_out := replace(text_out, '@BSP@', round((metadata->>'speed')::numeric * 3600 / 1852, 1)::text);
+			END IF;
+		END IF;
+
+		IF position('@WD@' IN text_out) != 0 THEN
+			IF metadata ? 'waterDepth' THEN
+				text_out := replace(text_out, '@WD@', metadata->>'waterDepth');
+			END IF;
+		END IF;
+
+		json_query := (regexp_match(text_out, '@(\$\..*?)@@'))[1];
+		IF json_query IS NOT NULL THEN
+			json_result := jsonb_path_query_array(metadata, json_query::jsonpath);
+			IF jsonb_array_length(json_result) = 1 THEN
+				text_out := replace(text_out, '@'||json_query||'@@', json_result->>0);
+			ELSE
+				text_out := replace(text_out, '@'||json_query||'@@', json_result::text);
+			END IF;
+			-- There might be multiple JSONPath queries, so we may have to recurse
+			expect_recursion := true;
+		END IF;
+
+		IF expect_recursion IS TRUE AND text_in != text_out THEN
+			--RAISE NOTICE 'Recursing %', text_out;
+			-- We don't know if we have found all the JSONPath expression
+			-- so we do another pass.
+			RETURN replace_placeholders(text_out, tstamp, sequence, point);
+		ELSE
+			RETURN text_out;
+		END IF;
+
+	END;
+	$$ LANGUAGE plpgsql;
+
+	COMMENT ON FUNCTION replace_placeholders (text, timestamptz, integer, integer) IS
+	'Replace certain placeholder strings in the input text with data obtained from shot or real-time data.';
+
+
+	CREATE OR REPLACE PROCEDURE scan_placeholders ()
+	LANGUAGE sql
+	AS $$
+		-- We update non read-only events via the event_log view to leave a trace
+		-- of the fact that placeholders were replaced (and when).
+		-- Note that this will not replace placeholders of old edits.
+		UPDATE event_log
+		SET remarks = replace_placeholders(remarks, tstamp, sequence, point)
+		FROM (
+			SELECT id
+			FROM event_log e
+			WHERE
+				(meta->'readonly')::boolean IS NOT TRUE AND (
+					regexp_match(remarks, '@DMS@|@POS@|@DEG@') IS NOT NULL OR
+					regexp_match(remarks, '@BSP@|@WD@|@CMG@|@EN@|@GRID@|@(\$\..*?)@@') IS NOT NULL
+				)
+		) t
+		WHERE event_log.id = t.id;
+
+		-- And then we update read-only events directly on the event_log_full table
+		-- (as of this version of the schema we're prevented from updating read-only
+		-- events via event_log anyway).
+		UPDATE event_log_full
+		SET remarks = replace_placeholders(remarks, tstamp, sequence, point)
+		FROM (
+			SELECT uid
+			FROM event_log_full e
+			WHERE
+				(meta->'readonly')::boolean IS TRUE AND (
+					regexp_match(remarks, '@DMS@|@POS@|@DEG@') IS NOT NULL OR
+					regexp_match(remarks, '@BSP@|@WD@|@CMG@|@EN@|@GRID@|@(\$\..*?)@@') IS NOT NULL
+				)
+		) t
+		WHERE event_log_full.uid = t.uid;
+	$$;
+
+	COMMENT ON PROCEDURE scan_placeholders () IS
+	'Run replace_placeholders() on the entire event log.';
+
+END;
+$outer$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE PROCEDURE pg_temp.upgrade () AS $outer$
+DECLARE
+	row RECORD;
+BEGIN
+
+	FOR row IN
+		SELECT schema_name FROM information_schema.schemata
+		WHERE schema_name LIKE 'survey_%'
+		ORDER BY schema_name
+	LOOP
+		CALL pg_temp.upgrade_survey_schema(row.schema_name);
+	END LOOP;
+END;
+$outer$ LANGUAGE plpgsql;
+
+CALL pg_temp.upgrade();
+
+CALL show_notice('Cleaning up');
+DROP PROCEDURE pg_temp.upgrade_survey_schema (schema_name text);
+DROP PROCEDURE pg_temp.upgrade ();
+
+CALL show_notice('Updating db_schema version');
+INSERT INTO public.info VALUES ('version', '{"db_schema": "0.3.9"}')
+ON CONFLICT (key) DO UPDATE
+	SET value = public.info.value || '{"db_schema": "0.3.9"}' WHERE public.info.key = 'version';
+
+
+CALL show_notice('All done. You may now run "COMMIT;" to persist the changes');
+DROP PROCEDURE show_notice (notice text);
+
+--
+--NOTE Run `COMMIT;` now if all went well
+--