_localOffsetForDate() was broken

It was including the elapsed time of day when calculating
the offset, leading to incorrect results

proto/backend.proto

@@ -15,6 +15,7 @@ message BackendInput {
         FindCardsIn find_cards = 19;
         BrowserRowsIn browser_rows = 20;
         RenderCardIn render_card = 21;
+        int64 local_minutes_west = 22;
 
         PlusOneIn plus_one = 2046; // temporary, for testing
     }
@@ -28,6 +29,7 @@ message BackendOutput {
         FindCardsOut find_cards = 19;
         BrowserRowsOut browser_rows = 20;
         RenderCardOut render_card = 21;
+        sint32 local_minutes_west = 22;
 
         PlusOneOut plus_one = 2046; // temporary, for testing
 

pylib/anki/collection.py

@@ -132,7 +132,7 @@ class _Collection:
         elif ver == 2:
             self.sched = V2Scheduler(self)
             if not self.server:
-                self.conf["localOffset"] = self.sched.currentTimezoneOffset()
+                self.conf["localOffset"] = self.sched._current_timezone_offset()
             elif self.server.minutes_west is not None:
                 self.conf["localOffset"] = self.server.minutes_west
 
@@ -162,7 +162,7 @@ class _Collection:
         if isinstance(self.sched, V1Scheduler):
             return None
         else:
-            return self.sched.currentTimezoneOffset()
+            return self.sched._current_timezone_offset()
 
     # DB-related
     ##########################################################################

pylib/anki/rsbackend.py

@@ -143,3 +143,8 @@ class RustBackend:
         anodes = proto_replacement_list_to_native(out.answer_nodes)  # type: ignore
 
         return (qnodes, anodes)
+
+    def local_minutes_west(self, stamp: int) -> int:
+        return self._run_command(
+            pb.BackendInput(local_minutes_west=stamp)
+        ).local_minutes_west

pylib/anki/schedv2.py

@@ -1340,9 +1340,9 @@ where id = ?
 
     def _updateCutoff(self) -> None:
         oldToday = self.today
-        timing = self._timingToday()
+        timing = self._timing_today()
 
-        if self._newTimezoneEnabled():
+        if self._new_timezone_enabled():
             self.today = timing.days_elapsed
             self.dayCutoff = timing.next_day_at
         else:
@@ -1398,41 +1398,40 @@ where id = ?
     # New timezone handling
     ##########################################################################
 
-    def _newTimezoneEnabled(self) -> bool:
+    def _new_timezone_enabled(self) -> bool:
         return self.col.conf.get("creationOffset") is not None
 
-    def _timingToday(self) -> SchedTimingToday:
+    def _timing_today(self) -> SchedTimingToday:
         return self.col.backend.sched_timing_today(
             self.col.crt,
-            self.creationTimezoneOffset(),
+            self._creation_timezone_offset(),
             intTime(),
-            self.currentTimezoneOffset(),
+            self._current_timezone_offset(),
             self._rolloverHour(),
         )
 
-    def currentTimezoneOffset(self) -> int:
+    def _current_timezone_offset(self) -> int:
         if self.col.server:
             return self.col.conf.get("localOffset", 0)
         else:
-            return self._localOffsetForDate(datetime.datetime.today())
+            return self.col.backend.local_minutes_west(intTime())
 
-    def creationTimezoneOffset(self) -> int:
+    def _creation_timezone_offset(self) -> int:
         return self.col.conf.get("creationOffset", 0)
 
-    def setCreationOffset(self):
+    def set_creation_offset(self):
         """Save the UTC west offset at the time of creation into the DB.
 
         Once stored, this activates the new timezone handling code.
         """
-        mins_west = self._localOffsetForDate(
-            datetime.datetime.fromtimestamp(self.col.crt)
-        )
+        mins_west = self.col.backend.local_minutes_west(self.col.crt)
         self.col.conf["creationOffset"] = mins_west
         self.col.setMod()
 
-    def _localOffsetForDate(self, date: datetime.datetime) -> int:
-        "Minutes west of UTC for a given datetime in the local timezone."
-        return date.astimezone().utcoffset().seconds // -60
+    def clear_creation_offset(self):
+        if "creationOffset" in self.col.conf:
+            del self.col.conf["creationOffset"]
+            self.col.setMod()
 
     # Deck finished state
     ##########################################################################

rslib/src/backend.rs

@@ -5,7 +5,7 @@ use crate::backend_proto as pt;
 use crate::backend_proto::backend_input::Value;
 use crate::backend_proto::RenderedTemplateReplacement;
 use crate::err::{AnkiError, Result};
-use crate::sched::sched_timing_today;
+use crate::sched::{local_minutes_west_for_stamp, sched_timing_today};
 use crate::template::{
     render_card, without_legacy_template_directives, FieldMap, FieldRequirements, ParsedTemplate,
     RenderedNode,
@@ -96,6 +96,9 @@ impl Backend {
             Value::FindCards(_) => todo!(),
             Value::BrowserRows(_) => todo!(),
             Value::RenderCard(input) => OValue::RenderCard(self.render_template(input)?),
+            Value::LocalMinutesWest(stamp) => {
+                OValue::LocalMinutesWest(local_minutes_west_for_stamp(stamp))
+            }
         })
     }
 

rslib/src/sched.rs

@@ -82,21 +82,21 @@ fn fixed_offset_from_minutes(minutes_west: i32) -> FixedOffset {
     FixedOffset::west(bounded_minutes * 60)
 }
 
-/// Relative to the local timezone, the number of minutes UTC differs by.
+/// For the given timestamp, return minutes west of UTC in the
+/// local timezone.
 /// eg, Australia at +10 hours is -600.
 /// Includes the daylight savings offset if applicable.
-#[allow(dead_code)]
-fn utc_minus_local_mins() -> i32 {
-    Local::now().offset().utc_minus_local() / 60
+pub fn local_minutes_west_for_stamp(stamp: i64) -> i32 {
+    Local.timestamp(stamp, 0).offset().utc_minus_local() / 60
 }
 
 #[cfg(test)]
 mod test {
     use crate::sched::{
-        fixed_offset_from_minutes, normalized_rollover_hour, sched_timing_today,
-        utc_minus_local_mins,
+        fixed_offset_from_minutes, local_minutes_west_for_stamp, normalized_rollover_hour,
+        sched_timing_today,
     };
-    use chrono::{FixedOffset, Local, TimeZone};
+    use chrono::{FixedOffset, Local, TimeZone, Utc};
 
     #[test]
     fn test_rollover() {
@@ -121,9 +121,16 @@ mod test {
         today.days_elapsed
     }
 
+    #[test]
+    fn test_local_minutes_west() {
+        // -480 throughout the year
+        std::env::set_var("TZ", "Australia/Perth");
+        assert_eq!(local_minutes_west_for_stamp(Utc::now().timestamp()), -480);
+    }
+
     #[test]
     fn test_days_elapsed() {
-        let local_offset = utc_minus_local_mins();
+        let local_offset = local_minutes_west_for_stamp(Utc::now().timestamp());
 
         let created_dt = FixedOffset::west(local_offset * 60)
             .ymd(2019, 12, 1)