Replace the age example in the README and document datetime columns

Use the release year from Neo4j's Movies dataset for the typed column
example, and show how to turn a Unix timestamp and an ISO 8601 string
into timestamp and timestamptz generated columns.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -89,25 +89,63 @@ returns a property as text. For typed columns, add them yourself, either as a vi
 as a generated column:
 
 ```sql
-CREATE VIEW neo4j.people AS
-    SELECT id, properties ->> 'name'::text AS name, (properties ->> 'age'::text)::bigint AS age
-    FROM neo4j."Person";
+CREATE VIEW neo4j.movies AS
+    SELECT id, properties ->> 'title'::text AS title, (properties ->> 'released'::text)::integer AS released
+    FROM neo4j."Movie";
 
-ALTER TABLE neo4j."Person"
-    ADD COLUMN age bigint GENERATED ALWAYS AS ((properties ->> 'age'::text)::bigint) STORED;
+ALTER TABLE neo4j."Movie"
+    ADD COLUMN released integer GENERATED ALWAYS AS ((properties ->> 'released'::text)::integer) STORED;
 ```
 
 PostgreSQL keeps generated columns up to date on every Cypher `CREATE`, `MERGE`, `SET`
 and `REMOVE`, and they can be indexed. Some things to keep in mind:
 
 - A value that does not fit the column's type makes the Cypher write fail: with the
-  column above, `CREATE (:Person {age: 'old'})` is rejected. A view only fails when it
+  column above, `CREATE (:Movie {released: 'soon'})` is rejected. A view only fails when it
   is read.
 - AGE does not allow dropping columns from its label tables, so a generated column
   cannot be removed again.
 - Adding a stored generated column rewrites the table and locks it while doing so.
-- AGE has no date or time types, so store those as ISO 8601 strings and cast them in
-  SQL.
+
+### Dates and times
+
+AGE has no date or time types, so store a point in time either as a Unix timestamp
+(seconds since 1970-01-01 UTC) or as an ISO 8601 string, and convert it in SQL. A Unix
+timestamp converts with the built-in `to_timestamp`, here to a `timestamp` in UTC:
+
+```sql
+ALTER TABLE neo4j."Movie"
+    ADD COLUMN updated timestamp GENERATED ALWAYS AS
+        (to_timestamp((properties ->> 'updated'::text)::bigint) AT TIME ZONE 'UTC') STORED;
+```
+
+A generated column only accepts immutable functions, and casting text to `timestamptz`
+is not immutable, because the result depends on the `TimeZone` and `DateStyle`
+settings. A view can use the cast as is, but a generated column needs a function that
+only accepts strings with a UTC offset and parses them the same way whatever the
+session settings are:
+
+```sql
+CREATE FUNCTION public.iso8601_timestamptz(value text) RETURNS timestamptz
+    LANGUAGE plpgsql IMMUTABLE STRICT PARALLEL SAFE
+    SET TimeZone = 'UTC'
+AS $$
+BEGIN
+    IF value !~ '^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}(:\d{2}(\.\d+)?)?(Z|[+-]\d{2}(:?\d{2})?)$' THEN
+        RAISE EXCEPTION 'not an ISO 8601 timestamp with a UTC offset: %', value;
+    END IF;
+    RETURN value::timestamptz;
+END
+$$;
+
+ALTER TABLE neo4j."Movie"
+    ADD COLUMN premiere timestamptz GENERATED ALWAYS AS
+        (public.iso8601_timestamptz(properties ->> 'premiere'::text)) STORED;
+```
+
+With these columns, `CREATE (:Movie {title: 'The Matrix', updated: 1729500000,
+premiere: '1999-03-24T19:00:00-08:00'})` fills in both, and a `premiere` without an
+offset, such as `'1999-03-24T19:00:00'`, makes the write fail.
 
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