diff --git a/apps/api/src/app/account-balance/account-balance.service.ts b/apps/api/src/app/account-balance/account-balance.service.ts
index 321624003..d81dc5208 100644
--- a/apps/api/src/app/account-balance/account-balance.service.ts
+++ b/apps/api/src/app/account-balance/account-balance.service.ts
@@ -14,7 +14,7 @@ import { Injectable } from '@nestjs/common';
 import { EventEmitter2 } from '@nestjs/event-emitter';
 import { AccountBalance, Prisma } from '@prisma/client';
 import { Big } from 'big.js';
-import { format, parseISO } from 'date-fns';
+import { endOfDay, format, parseISO } from 'date-fns';
 
 @Injectable()
 export class AccountBalanceService {
@@ -109,11 +109,19 @@ export class AccountBalanceService {
       userId,
       withExcludedAccounts: false // TODO
     });
+
+    // Exclude future-dated balances so they do not affect portfolio calculation (fixes #6185)
+    const asOfDate = endOfDay(new Date());
+    const asOfTime = asOfDate.getTime();
+    const balancesUpToToday = balances.filter(({ date }) => {
+      return date.getTime() <= asOfTime;
+    });
+
     const accumulatedBalancesByDate: { [date: string]: HistoricalDataItem } =
       {};
     const lastBalancesByAccount: { [accountId: string]: Big } = {};
 
-    for (const { accountId, date, valueInBaseCurrency } of balances) {
+    for (const { accountId, date, valueInBaseCurrency } of balancesUpToToday) {
       const formattedDate = format(date, DATE_FORMAT);
 
       lastBalancesByAccount[accountId] = new Big(valueInBaseCurrency);
diff --git a/apps/api/src/app/order/order.service.ts b/apps/api/src/app/order/order.service.ts
index 9a4f1e46b..e9da1ade8 100644
--- a/apps/api/src/app/order/order.service.ts
+++ b/apps/api/src/app/order/order.service.ts
@@ -371,10 +371,16 @@ export class OrderService {
         filters: [{ id: account.id, type: 'ACCOUNT' }]
       });
 
+      // Exclude future-dated balances so they do not affect portfolio calculation (fixes #6185)
+      const asOfDate = endOfToday();
+      const balancesUpToToday = balances.filter(({ date }) => {
+        return date <= asOfDate;
+      });
+
       let currentBalance = 0;
       let currentBalanceInBaseCurrency = 0;
 
-      for (const balanceItem of balances) {
+      for (const balanceItem of balancesUpToToday) {
         const syntheticActivityTemplate: Activity = {
           userId,
           accountId: account.id,
diff --git a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-cash.spec.ts b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-cash.spec.ts
index a53ebcf05..77413cac1 100644
--- a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-cash.spec.ts
+++ b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-cash.spec.ts
@@ -157,6 +157,14 @@ describe('PortfolioCalculator', () => {
               date: parseDate('2024-12-31'),
               value: 2000,
               valueInBaseCurrency: 1800
+            },
+            {
+              // Future-dated balance: should be ignored (issue #6185)
+              accountId,
+              date: parseDate('2050-12-31'),
+              id: randomUUID(),
+              value: 0,
+              valueInBaseCurrency: 0
             }
           ]
         });