diff --git a/src/lib/format.ts b/src/lib/format.ts
index 4f16c76..8268ba7 100644
--- a/src/lib/format.ts
+++ b/src/lib/format.ts
@@ -22,3 +22,14 @@ export function formatDay(isoDay: string) {
   if (isNaN(parsed)) return isoDay;
   return new Date(parsed).toLocaleDateString(undefined, { dateStyle: "medium", timeZone: "UTC" });
 }
+
+/**
+ * Convert a UTC ISO date string into a local YYYY-MM-DDTHH:mm string
+ * suitable for a <input type="datetime-local">.
+ */
+export function toLocalDatetimeString(isoDate: string) {
+  const date = new Date(isoDate);
+  if (isNaN(date.getTime())) return "";
+  const tzOffset = date.getTimezoneOffset() * 60000;
+  return new Date(date.getTime() - tzOffset).toISOString().slice(0, 16);
+}
diff --git a/src/routes/group.$id/expense.$expenseId.tsx b/src/routes/group.$id/expense.$expenseId.tsx
index 2cec295..4bc4416 100644
--- a/src/routes/group.$id/expense.$expenseId.tsx
+++ b/src/routes/group.$id/expense.$expenseId.tsx
@@ -14,6 +14,7 @@ import {
 } from "@/hooks/useApi";
 import { useAppForm } from "@/hooks/useAppForm";
 import { expenseFormDefaults } from "@/lib/expense-form";
+import { toLocalDatetimeString } from "@/lib/format";
 import { pb } from "@/lib/pocketbase";
 import { requireAuth } from "@/lib/requireAuth";
 import { createFileRoute, Link, useNavigate } from "@tanstack/react-router";
@@ -47,7 +48,7 @@ function EditExpensePage() {
       title: expense?.title ?? "",
       amount: expense?.amount?.toString() ?? "",
       currency: expense?.currency || base,
-      date: expense?.date ? expense.date.slice(0, 16) : "",
+      date: expense?.date ? toLocalDatetimeString(expense.date) : "",
       paidBy: expense?.paidBy ?? currentUserId,
       category: expense?.category ?? "",
       project: expense?.project ?? "",
@@ -67,7 +68,10 @@ function EditExpensePage() {
     },
     onSubmit: async ({ value }) => {
       const amount = parseFloat(value.amount);
-      const date = new Date(value.date).toISOString();
+      const date =
+        expense?.date && value.date === toLocalDatetimeString(expense.date)
+          ? expense.date
+          : new Date(value.date).toISOString();
 
       // Keep the originally locked rate unless the amount or currency changed;
       // otherwise re-convert at the rate effective on the expense's date.