"""ISO-8601 UTC helpers. All schedule instants are tz-aware UTC, formatted with
a trailing 'Z' so golden fixtures compare exactly."""

from datetime import date, datetime, timezone
from zoneinfo import ZoneInfo


def parse_start(value: str, tz_name: str = "UTC") -> datetime:
    """Accepts 'YYYY-MM-DD' or full ISO (optionally trailing 'Z').

    A bare date (or a datetime without an explicit offset) is interpreted in
    ``tz_name`` (the project's calendar timezone), so '2026-07-01' for a Dubai
    project means local midnight, not UTC midnight. An explicit offset/`Z` wins.
    """
    v = value.strip()
    if len(v) == 10:
        return datetime.fromisoformat(v).replace(tzinfo=ZoneInfo(tz_name)).astimezone(timezone.utc)
    dt = datetime.fromisoformat(v.replace("Z", "+00:00"))
    if dt.tzinfo is None:
        dt = dt.replace(tzinfo=ZoneInfo(tz_name))
    return dt.astimezone(timezone.utc)


def to_iso_z(dt: datetime) -> str:
    return dt.astimezone(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")


def today_iso() -> str:
    return date.today().isoformat()