Simplify status interface

solar_relay/config.py

@@ -1,7 +1,7 @@
 """Private runtime configuration; credential fields are excluded from reprs."""
 
 from dataclasses import dataclass, field
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
 import math
 from pathlib import Path
 import re
@@ -18,6 +18,15 @@ def utc_text(value: datetime) -> str:
     return value.astimezone(timezone.utc).isoformat(timespec="milliseconds").replace("+00:00", "Z")
 
 
+def retention_cutoff(now: datetime, days: int, timezone_name: str) -> datetime:
+    """Start of the oldest retained local calendar day, returned in UTC."""
+    if now.tzinfo is None:
+        raise ValueError("Retention time must include its timezone")
+    zone = ZoneInfo(timezone_name)
+    first = now.astimezone(zone).date() - timedelta(days=days)
+    return datetime.combine(first, datetime.min.time(), zone).astimezone(timezone.utc)
+
+
 def date_bound(value: str | None, timezone_name: str) -> str | None:
     """Date bounds are local midnights; timestamp bounds may specify an offset."""
     if value is None: