annotate duration with Second type alias

my/location/fallback/common.py

@@ -6,12 +6,14 @@ from datetime import datetime, timedelta, timezone
 from ..common import LocationProtocol, Location
 DateExact = Union[datetime, float, int]  # float/int as epoch timestamps
 
+Second = float
+
 @dataclass
 class FallbackLocation(LocationProtocol):
     lat: float
     lon: float
     dt: datetime
-    duration: Optional[float] = None # time in seconds for how long this is valid
+    duration: Optional[Second] = None
     accuracy: Optional[float] = None
     elevation: Optional[float] = None
     datasource: Optional[str] = None  # which module provided this, useful for debugging

my/time/tz/via_location.py

@@ -250,6 +250,13 @@ def _get_tz(dt: datetime) -> Optional[pytz.BaseTzInfo]:
     if res is not None:
         return res
     # fallback to home tz
+    # note: the fallback to fallback.via_home.estimate_location is still needed, since
+    # _iter_local_dates_fallback only returns days which we actually have a datetime for
+    # (e.g. there was an IP address within a day of that datetime)
+    #
+    # given a datetime, fallback.via_home.estimate_location will find which home location
+    # that datetime is between, else fallback on your first home location, so it acts
+    # as a last resort
     from my.location.fallback import via_home as home
     loc = list(home.estimate_location(dt))
     assert len(loc) == 1, f"should only have one home location, received {loc}"