diff --git a/examples/optimize.js b/examples/optimize.js
index 01af63c..b36a7e9 100644
--- a/examples/optimize.js
+++ b/examples/optimize.js
@@ -38,30 +38,124 @@ function createReviewHistoriesForCards() {
 
   const reviewHistories = [
     [
-      [new Date('2023-01-01'), 3],
-      [new Date('2023-01-02'), 4],
-      [new Date('2023-01-05'), 3],
-      [new Date('2023-01-15'), 4],
-      [new Date('2023-02-01'), 3],
-      [new Date('2023-02-20'), 4],
+      [new Date("2023-01-01"), 3],
+      [new Date("2023-01-02"), 4],
+      [new Date("2023-01-05"), 3],
+      [new Date("2023-01-15"), 4],
+      [new Date("2023-02-01"), 3],
+      [new Date("2023-02-20"), 4],
     ],
     [
-      [new Date('2023-01-01'), 2],
-      [new Date('2023-01-02'), 3],
-      [new Date('2023-01-04'), 4],
-      [new Date('2023-01-12'), 3],
-      [new Date('2023-01-28'), 4],
-      [new Date('2023-02-15'), 3],
-      [new Date('2023-03-05'), 4],
+      [new Date("2023-01-01"), 2],
+      [new Date("2023-01-02"), 3],
+      [new Date("2023-01-04"), 4],
+      [new Date("2023-01-12"), 3],
+      [new Date("2023-01-28"), 4],
+      [new Date("2023-02-15"), 3],
+      [new Date("2023-03-05"), 4],
     ],
     [
-      [new Date('2023-01-01'), 4],
-      [new Date('2023-01-08'), 4],
-      [new Date('2023-01-24'), 3],
-      [new Date('2023-02-10'), 4],
-      [new Date('2023-03-01'), 3],
+      [new Date("2023-01-01"), 4],
+      [new Date("2023-01-08"), 4],
+      [new Date("2023-01-24"), 3],
+      [new Date("2023-02-10"), 4],
+      [new Date("2023-03-01"), 3],
+    ],
+    [
+      [new Date("2023-01-01"), 1],
+      [new Date("2023-01-02"), 1],
+      [new Date("2023-01-03"), 3],
+      [new Date("2023-01-06"), 4],
+      [new Date("2023-01-16"), 4],
+      [new Date("2023-02-01"), 3],
+      [new Date("2023-02-20"), 4],
+    ],
+    [
+      [new Date("2023-01-01"), 3],
+      [new Date("2023-01-03"), 3],
+      [new Date("2023-01-08"), 2],
+      [new Date("2023-01-10"), 4],
+      [new Date("2023-01-22"), 3],
+      [new Date("2023-02-05"), 4],
+      [new Date("2023-02-25"), 3],
+    ],
+    [
+      [new Date("2023-01-01"), 4],
+      [new Date("2023-01-09"), 3],
+      [new Date("2023-01-19"), 4],
+      [new Date("2023-02-05"), 3],
+      [new Date("2023-02-25"), 4],
+    ],
+    [
+      [new Date("2023-01-01"), 2],
+      [new Date("2023-01-02"), 3],
+      [new Date("2023-01-05"), 4],
+      [new Date("2023-01-15"), 3],
+      [new Date("2023-01-30"), 4],
+      [new Date("2023-02-15"), 3],
+      [new Date("2023-03-05"), 4],
+    ],
+    [
+      [new Date("2023-01-01"), 3],
+      [new Date("2023-01-04"), 4],
+      [new Date("2023-01-14"), 4],
+      [new Date("2023-02-01"), 3],
+      [new Date("2023-02-20"), 4],
+    ],
+    [
+      [new Date("2023-01-01"), 1],
+      [new Date("2023-01-01"), 3],
+      [new Date("2023-01-02"), 1],
+      [new Date("2023-01-02"), 3],
+      [new Date("2023-01-03"), 3],
+      [new Date("2023-01-07"), 3],
+      [new Date("2023-01-15"), 4],
+      [new Date("2023-01-31"), 3],
+      [new Date("2023-02-15"), 4],
+      [new Date("2023-03-05"), 3],
+    ],
+    [
+      [new Date("2023-01-01"), 4],
+      [new Date("2023-01-10"), 3],
+      [new Date("2023-01-20"), 4],
+      [new Date("2023-02-05"), 4],
+      [new Date("2023-02-25"), 3],
+      [new Date("2023-03-15"), 4],
+    ],
+    [
+      [new Date("2023-01-01"), 1],
+      [new Date("2023-01-02"), 2],
+      [new Date("2023-01-03"), 3],
+      [new Date("2023-01-04"), 4],
+      [new Date("2023-01-10"), 3],
+      [new Date("2023-01-20"), 4],
+      [new Date("2023-02-05"), 3],
+      [new Date("2023-02-25"), 4],
+    ],
+    [
+      [new Date("2023-01-01"), 3],
+      [new Date("2023-01-05"), 4],
+      [new Date("2023-01-15"), 3],
+      [new Date("2023-01-30"), 4],
+      [new Date("2023-02-15"), 3],
+      [new Date("2023-03-05"), 4],
+    ],
+    [
+      [new Date("2023-01-01"), 2],
+      [new Date("2023-01-03"), 3],
+      [new Date("2023-01-07"), 4],
+      [new Date("2023-01-17"), 3],
+      [new Date("2023-02-01"), 4],
+      [new Date("2023-02-20"), 3],
+      [new Date("2023-03-10"), 4],
+    ],
+    [
+      [new Date("2023-01-01"), 4],
+      [new Date("2023-01-12"), 3],
+      [new Date("2023-01-25"), 4],
+      [new Date("2023-02-10"), 3],
+      [new Date("2023-03-01"), 4],
     ],
-    // ... add more review history arrays
   ]
   // Cycle and repeat the array to create a larger dataset of 100 cards
   const res = []
@@ -85,7 +179,7 @@ function convertToFSRSItem(history) {
 
 
   for (const [date, rating] of history) {
-    const deltaT = (date.getTime() - lastDate.getTime()) / (1000 * 60 * 60 * 24) // Calculate difference in days
+    const deltaT = dateDiffInDays(lastDate, date)
     reviews.push(new FSRSReview(rating, deltaT))
     items.push(new FSRSItem([...reviews]))
     lastDate = date
@@ -94,4 +188,13 @@ function convertToFSRSItem(history) {
   return items.filter((item) => item.longTermReviewCnt() > 0)
 }
 
+function dateDiffInDays(a, b) {
+  const _MS_PER_DAY = 1000 * 60 * 60 * 24;
+  // Discard the time and time-zone information.
+  const utc1 = Date.UTC(a.getFullYear(), a.getMonth(), a.getDate());
+  const utc2 = Date.UTC(b.getFullYear(), b.getMonth(), b.getDate());
+
+  return Math.floor((utc2 - utc1) / _MS_PER_DAY)
+}
+
 main()