Give it a longitude and latitude and it answers with the IANA timezone identifier (America/New_York), the UTC offset in force there right now, whether daylight saving is currently in effect and by how much, the local wall-clock time, and the zone's standard offset. Pass an optional ISO 8601 `at` instant to ask about any moment between 1900 and 2100 instead of now, which is what you want when scheduling something months out across a DST boundary. Parameters go in the query string: this is a GET, there is no body. Two halves, with different vintages, and the difference matters. The offset and the DST rules come from the runtime's own timezone database and are current: a rule change reaches this endpoint when Cloudflare updates ICU, with no redeploy here. The *boundaries* come from tz_world (efele.net), CC0 1.0 and derived from VMAP0, a public-domain US government product. That map was frozen at tzdb 2016d in May 2016, and this catalog sells public-domain and permissively-licensed data on