diff --git a/common/changes/@visactor/vtable/fix-gantt-time-scaleCount_2026-01-13-09-13.json b/common/changes/@visactor/vtable/fix-gantt-time-scaleCount_2026-01-13-09-13.json
new file mode 100644
index 0000000000..45d2859635
--- /dev/null
+++ b/common/changes/@visactor/vtable/fix-gantt-time-scaleCount_2026-01-13-09-13.json
@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "fix: time unit computeCountToTimeScale month exactitude #4909\n\n",
+      "type": "none",
+      "packageName": "@visactor/vtable"
+    }
+  ],
+  "packageName": "@visactor/vtable",
+  "email": "892739385@qq.com"
+}
\ No newline at end of file
diff --git a/packages/vtable-gantt/src/tools/util.ts b/packages/vtable-gantt/src/tools/util.ts
index 21716b6a12..c906034593 100644
--- a/packages/vtable-gantt/src/tools/util.ts
+++ b/packages/vtable-gantt/src/tools/util.ts
@@ -677,6 +677,7 @@ export function computeCountToTimeScale(
 
   let difference: number;
   const adjusted_date = new Date(date.getTime() + diffMS);
+
   switch (timeScale) {
     case 'second':
       difference = (adjusted_date.getTime() - startDate.getTime()) / msInSecond;
@@ -694,12 +695,15 @@ export function computeCountToTimeScale(
       difference = (adjusted_date.getTime() - startDate.getTime()) / msInWeek;
       break;
     case 'month':
+      const startDaysInMonth = new Date(startDate.getFullYear(), startDate.getMonth() + 1, 0).getDate();
+      const adjustedDaysInMonth = new Date(adjusted_date.getFullYear(), adjusted_date.getMonth() + 1, 0).getDate();
       difference =
         (adjusted_date.getFullYear() - startDate.getFullYear()) * 12 +
         (adjusted_date.getMonth() - startDate.getMonth());
-      difference +=
-        (adjusted_date.getDate() - startDate.getDate()) /
-        new Date(adjusted_date.getFullYear(), adjusted_date.getMonth() + 1, 0).getDate();
+
+      // Calculate fractional difference by normalizing day progress within each specific month
+      // This handles variable month lengths (28/29/30/31 days) correctly
+      difference += adjusted_date.getDate() / adjustedDaysInMonth - startDate.getDate() / startDaysInMonth;
       break;
     case 'quarter':
       difference =