Fix Cron.next skipping days when overflowing

.changeset/fix-cron-next-missing-day-overflow.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Fix `Cron.next` skipping earlier matching days when the upcoming day-of-month does not exist in the current month.

packages/effect/src/Cron.ts

@@ -868,6 +868,10 @@ const stepCron = (cron: Cron, now: DateTime.DateTime.Input | undefined, directio
             } else {
               b = daysInMonth(current) - currentDay + boundary.day
             }
+          } else if (!reverse && nextDay > daysInMonth(current)) {
+            // The next matching day does not exist in the current month. Setting it
+            // directly would overflow and skip earlier matching days next month.
+            b = daysInMonth(current) - currentDay + boundary.day
           } else {
             b = nextDay - currentDay
           }

packages/effect/test/Cron.test.ts

@@ -169,6 +169,21 @@ describe("Cron", () => {
     deepStrictEqual(next(Cron.parseUnsafe("5 0 8 2 *", london), after), DateTime.toDateUtc(amsterdamTime))
   })
 
+  it("next does not skip earlier days when the upcoming day is missing from the month", () => {
+    const tz = DateTime.zoneMakeNamedUnsafe("UTC")
+    const cron = Cron.parseUnsafe("0 0 1,16,31 * *", tz)
+    // From Feb 18 the next matching day in February would be the 31st, which does
+    // not exist. Rolling onto it must not overshoot past March 1 (also a match).
+    deepStrictEqual(next(cron, new Date("2020-02-18T00:00:00.000Z")), new Date("2020-03-01T00:00:00.000Z"))
+    // `*/15` expands to days [1, 16, 31] and exhibits the same wrap.
+    deepStrictEqual(
+      next(Cron.parseUnsafe("0 0 */15 * *", tz), new Date("2020-02-18T00:00:00.000Z")),
+      new Date("2020-03-01T00:00:00.000Z")
+    )
+    // 30-day month: from the 20th the next day is the 31st (missing) -> July 1.
+    deepStrictEqual(next(cron, new Date("2024-06-20T00:00:00.000Z")), new Date("2024-07-01T00:00:00.000Z"))
+  })
+
   it("prev", () => {
     const utc = DateTime.zoneMakeNamedUnsafe("UTC")
     const before = new Date("2024-01-04T16:21:00Z")