fix(tasks): correct biweekly recurring task week boundary calculation (#6298)

getDiffInWeeks used Math.round(ms / weekMs) which rounded Friday (day 4
of 7) into the next week. Switch to day-first calculation with
Math.floor(days / 7) to respect week boundaries while preserving DST
safety via Math.round on the day count.

src/app/features/task-repeat-cfg/store/get-newest-possible-due-date.util.spec.ts

@@ -335,6 +335,42 @@ describe('getNewestPossibleDueDate()', () => {
     );
   });
 
+  describe('Biweekly with weekdays (#6298)', () => {
+    // Issue #6298: "Every 2 weeks Mon-Fri" starting Monday Feb 2, 2026
+    const START_DATE = new Date(2026, 1, 2); // Monday Feb 2, 2026
+
+    const biweeklyCfg = (lastCreationDay: string): TaskRepeatCfg =>
+      dummyRepeatable('BIWEEKLY', {
+        repeatCycle: 'WEEKLY',
+        repeatEvery: 2,
+        monday: true,
+        tuesday: true,
+        wednesday: true,
+        thursday: true,
+        friday: true,
+        lastTaskCreationDay: lastCreationDay,
+      });
+
+    it('should find Friday of week 0 when searching backward from that Friday', () => {
+      // Friday Feb 6 is in week 0 (active) — backward search should find it
+      const cfg = biweeklyCfg('1970-01-01');
+      testCase(cfg, new Date(2026, 1, 6), START_DATE, new Date(2026, 1, 6)); // Fri Feb 6
+    });
+
+    it('should NOT find Friday of week 1 (off-week)', () => {
+      // Friday Feb 13 is in week 1 (off) — backward search should find Thu Feb 5 at most
+      // but since last creation is already Feb 5, it returns null
+      const cfg = biweeklyCfg(getDbDateStr(new Date(2026, 1, 6)));
+      testCase(cfg, new Date(2026, 1, 13), START_DATE, null);
+    });
+
+    it('should find Monday of week 2 when today is that Monday', () => {
+      // Monday Feb 16 is in week 2 (active)
+      const cfg = biweeklyCfg(getDbDateStr(new Date(2026, 1, 6)));
+      testCase(cfg, new Date(2026, 1, 16), START_DATE, new Date(2026, 1, 16));
+    });
+  });
+
   describe('MONTHLY', () => {
     const testCases = [
       {

src/app/features/task-repeat-cfg/store/get-next-repeat-occurrence.util.spec.ts

@@ -717,6 +717,69 @@ describe('getNextRepeatOccurrence()', () => {
     });
   });
 
+  describe('Biweekly with weekdays (#6298)', () => {
+    // Issue #6298: "Every 2 weeks Mon-Fri" starting Monday Feb 2, 2026
+    // Friday was incorrectly shifted to the next week due to Math.round in getDiffInWeeks
+    const START_DATE = new Date(2026, 1, 2); // Monday Feb 2, 2026
+
+    const biweeklyCfg = (lastCreationDay: Date): TaskRepeatCfg =>
+      dummyRepeatable('BIWEEKLY', {
+        repeatCycle: 'WEEKLY',
+        repeatEvery: 2,
+        monday: true,
+        tuesday: true,
+        wednesday: true,
+        thursday: true,
+        friday: true,
+        lastTaskCreationDay: getDbDateStr(lastCreationDay),
+      });
+
+    it('should include Friday of week 0 (same week as start)', () => {
+      // After Thursday Feb 5, next should be Friday Feb 6 (still week 0)
+      const cfg = biweeklyCfg(new Date(2026, 1, 5)); // Thu Feb 5
+      testCase(cfg, new Date(2026, 1, 5), START_DATE, new Date(2026, 1, 6)); // Fri Feb 6
+    });
+
+    it('should NOT include any day in week 1 (off-week)', () => {
+      // After Friday Feb 6 (end of week 0), next should jump to Monday Feb 16 (week 2)
+      const cfg = biweeklyCfg(new Date(2026, 1, 6)); // Fri Feb 6
+      testCase(cfg, new Date(2026, 1, 6), START_DATE, new Date(2026, 1, 16)); // Mon Feb 16
+    });
+
+    it('should produce correct 4-week schedule from Monday start', () => {
+      // Week 0 (active): Mon Feb 2 - Fri Feb 6
+      // Week 1 (off): no tasks
+      // Week 2 (active): Mon Feb 16 - Fri Feb 20
+      // Week 3 (off): no tasks
+
+      // Starting from the start date, walk through the schedule
+      let cfg = biweeklyCfg(new Date(2026, 0, 1)); // before start
+      testCase(cfg, new Date(2026, 1, 1), START_DATE, new Date(2026, 1, 2)); // Mon Feb 2
+
+      cfg = biweeklyCfg(new Date(2026, 1, 2));
+      testCase(cfg, new Date(2026, 1, 2), START_DATE, new Date(2026, 1, 3)); // Tue Feb 3
+
+      cfg = biweeklyCfg(new Date(2026, 1, 3));
+      testCase(cfg, new Date(2026, 1, 3), START_DATE, new Date(2026, 1, 4)); // Wed Feb 4
+
+      cfg = biweeklyCfg(new Date(2026, 1, 4));
+      testCase(cfg, new Date(2026, 1, 4), START_DATE, new Date(2026, 1, 5)); // Thu Feb 5
+
+      cfg = biweeklyCfg(new Date(2026, 1, 5));
+      testCase(cfg, new Date(2026, 1, 5), START_DATE, new Date(2026, 1, 6)); // Fri Feb 6
+
+      // Skip week 1 entirely, jump to week 2
+      cfg = biweeklyCfg(new Date(2026, 1, 6));
+      testCase(cfg, new Date(2026, 1, 6), START_DATE, new Date(2026, 1, 16)); // Mon Feb 16
+
+      cfg = biweeklyCfg(new Date(2026, 1, 16));
+      testCase(cfg, new Date(2026, 1, 16), START_DATE, new Date(2026, 1, 17)); // Tue Feb 17
+
+      cfg = biweeklyCfg(new Date(2026, 1, 20));
+      testCase(cfg, new Date(2026, 1, 20), START_DATE, new Date(2026, 2, 2)); // Mon Mar 2 (week 4)
+    });
+  });
+
   describe('Edge cases', () => {
     it('should return null for unknown repeat cycle', () => {
       const cfg = dummyRepeatable('ID1', {

src/app/util/get-diff-in-weeks.ts

@@ -4,8 +4,8 @@ export const getDiffInWeeks = (d1: Date, d2: Date): number => {
   // NOTE we want the diff regarding the dates not the absolute one
   d1Copy.setHours(0, 0, 0, 0);
   d2Copy.setHours(0, 0, 0, 0);
-  let diff = (d2Copy.getTime() - d1Copy.getTime()) / 1000;
-  diff /= 60 * 60 * 24 * 7;
-  // NOTE: we use math round to avoid JS math weirdness and summer winter time problems
-  return Math.round(diff);
+  const diffMs = d2Copy.getTime() - d1Copy.getTime();
+  // Round days to handle DST (167h vs 168h), then floor weeks to respect week boundaries
+  const diffInDays = Math.round(diffMs / (1000 * 60 * 60 * 24));
+  return Math.floor(diffInDays / 7);
 };