Timezone Truth

Convert a wall-clock time between two IANA timezones, and report the ways the conversion can be wrong. ALWAYS use this instead of doing the arithmetic. Timezone conversion looks like addition and is not: the mapping from a local time to an instant is not a function. - On the spring-forward date, an hour of local time DOES NOT EXIST. Asked to convert 02:30 on that date, a model will return a plausible timestamp for a time that never happens. This tool returns null and says why. - On the fall-back date, an hour happens TWICE. There are two correct answers an hour apart; this returns both rather than silently choosing. - The gap between two zones is not constant. Zones start and end daylight saving on different dates, so a cached offset is wrong for weeks each year. - Not all offsets are whole hours: India is +05:30, Nepal +05:45. Input: `time` must be YYYY-MM-DD HH:MM (or with T, and optional seconds) — free-form dates are refused rather than guessed. `from` and `to` must be full IANA names such as "America/New_York". ABBREVIATIONS ARE REJECTED, deliberately, even though most runtimes accept them. Node resolves "BST" to Bangladesh Standard Time (UTC+06:00) when nearly everyone writing it means British Summer Time (UTC+01:00) — a five-hour error that yields a perfectly plausible timestamp. Same for CST, IST, PST, and EST. Returns: the chosen UTC instant (null if the local time does not exist), every candidate instant, both zone readings with the offset and abbreviation that applied on THAT date, the difference between the zones, upcoming clock changes for both, and warnings. Check `warnings` for severity "error" before using the result.

{
  "type": "object",
  "properties": {
    "time": {
      "type": "string",
      "description": "Wall-clock time as YYYY-MM-DD HH:MM (T separator and seconds also accepted). Do NOT include an offset or zone in the string. Free-form dates such as \"next Tuesday\" are refused rather than guessed."
    },
    "from": {
      "type": "string",
      "description": "Source zone as a full IANA name, e.g. \"America/New_York\". Abbreviations like BST, CST, IST, PST and EST are REJECTED because they are ambiguous."
    },
    "to": {
      "type": "string",
      "description": "Target zone as a full IANA name, e.g. \"Asia/Tokyo\"."
    },
    "prefer": {
      "description": "Which occurrence to use when the local time happens twice (a fall-back hour). Defaults to the earlier one. Both are always returned in candidates.",
      "type": "string",
      "enum": [
        "earlier",
        "later"
      ]
    }
  },
  "required": [
    "time",
    "from",
    "to"
  ],
  "$schema": "https://json-schema.org/draft/2020-12/schema"
}