fix: round the current date to whole seconds so occurrences on the end date are returned

src/CronExpression.ts

@@ -517,8 +517,15 @@ export class CronExpression {
     const dateMathVerb: DateMathOp = reverse ? DateMathOp.Subtract : DateMathOp.Add;
     const currentDate = new CronDate(this.#currentDate);
     const startTimestamp = currentDate.getTime();
-    let stepCount = 0;
 
+    if (currentDate.getMilliseconds() > 0) {
+      currentDate.setMilliseconds(0);
+      if (!reverse) {
+        currentDate.applyDateOperation(DateMathOp.Add, TimeUnit.Second, this.#fields.hour.values.length);
+      }
+    }
+
+    let stepCount = 0;
     while (++stepCount < LOOP_LIMIT) {
       this.#validateTimeSpan(currentDate);
 
@@ -543,15 +550,8 @@ export class CronExpression {
       }
 
       if (startTimestamp === currentDate.getTime()) {
-        // Still on the start time. Step one second in the search direction and keep
-        // looking so a distinct occurrence is returned. A backwards search that began
-        // on a sub-second offset is the exception: stripping the milliseconds below
-        // already yields an earlier matching time, so break and accept it instead of
-        // spinning until the loop limit.
-        if (dateMathVerb === 'Add' || currentDate.getMilliseconds() === 0) {
-          currentDate.applyDateOperation(dateMathVerb, TimeUnit.Second, this.#fields.hour.values.length);
-          continue;
-        }
+        currentDate.applyDateOperation(dateMathVerb, TimeUnit.Second, this.#fields.hour.values.length);
+        continue;
       }
       break;
     }
@@ -560,9 +560,6 @@ export class CronExpression {
       throw new Error(LOOPS_LIMIT_EXCEEDED_ERROR_MESSAGE);
     }
 
-    if (currentDate.getMilliseconds() !== 0) {
-      currentDate.setMilliseconds(0);
-    }
     this.#currentDate = currentDate;
     return currentDate;
   }

tests/CronExpression.test.ts

@@ -294,6 +294,15 @@ describe('CronExpression', () => {
       expect(() => cronExpression.prev()).toThrow(TIME_SPAN_OUT_OF_BOUNDS_ERROR_MESSAGE);
     });
 
+    test('should throw when the previous occurrence would land inside a sub-second start date', () => {
+      const cronExpression = CronExpressionParser.parse('* * * * * *', {
+        currentDate: new Date('2025-01-01T10:00:05.500Z'),
+        startDate: new Date('2025-01-01T10:00:05.200Z'),
+        tz: 'UTC',
+      });
+      expect(() => cronExpression.prev()).toThrow(TIME_SPAN_OUT_OF_BOUNDS_ERROR_MESSAGE);
+    });
+
     test('should throw when no occurrence exists within the loop limit', () => {
       // Day 31 restricted to April and June, which only ever have 30 days, so
       // the expression can never match and iteration must not return a bogus date.
@@ -305,6 +314,26 @@ describe('CronExpression', () => {
     });
   });
 
+  test('returns an occurrence on the end date when the current date carries milliseconds', () => {
+    const interval = CronExpressionParser.parse('*/1 * * * * *', {
+      currentDate: new Date('2025-01-01T00:00:09.151Z'),
+      endDate: new Date('2025-01-01T00:00:10.000Z'),
+      tz: 'UTC',
+    });
+    expect(interval.next().toISOString()).toBe('2025-01-01T00:00:10.000Z');
+  });
+
+  test('keeps interleaved forward and backward iteration anchored to the last result', () => {
+    const interval = CronExpressionParser.parse('*/5 * * * *', {
+      currentDate: new Date('2025-01-01T12:02:00.000Z'),
+      tz: 'UTC',
+    });
+    expect(interval.next().toISOString()).toBe('2025-01-01T12:05:00.000Z');
+    expect(interval.next().toISOString()).toBe('2025-01-01T12:10:00.000Z');
+    expect(interval.prev().toISOString()).toBe('2025-01-01T12:05:00.000Z');
+    expect(interval.next().toISOString()).toBe('2025-01-01T12:10:00.000Z');
+  });
+
   describe('stringify', () => {
     test('stringify cron expression all stars no seconds 0 * * * * *', () => {
       const expected = '0 * * * * *';