I think the comment is specifically talking about storing future times, and contemplating future changes to the local time zone offsets.
If I say that something is going to happen at noon local time on July 1, 2030 in New York, we know that is, under current rules, going to happen at 16:00 UTC. But what if the US changes its daylight savings rules between now and 2030? The canonical time for that event is noon local time, and the offset between local time and UTC can only certainly be determined with past events, so future events defined by local will necessarily have some uncertainty when it comes to UTC.
I think the comment is specifically talking about storing future times, and contemplating future changes to the local time zone offsets.
If I say that something is going to happen at noon local time on July 1, 2030 in New York, we know that is, under current rules, going to happen at 16:00 UTC. But what if the US changes its daylight savings rules between now and 2030? The canonical time for that event is noon local time, and the offset between local time and UTC can only certainly be determined with past events, so future events defined by local will necessarily have some uncertainty when it comes to UTC.