I get this information from the IANA timezone database shipped with pytz. With it you can easily detect when a TZ changes its time because of toggling DST on or off.

I was thinking about an API as well, I will probably add RSS, webhooks and a simple REST API.

The thing is, I'm not sure the API will be used much. Most systems already have access to the same TZ database I use, so why make an HTTP call to retrieve this information?