diff --git a/TransactionProcessor.BusinessLogic/Common/Extensions.cs b/TransactionProcessor.BusinessLogic/Common/Extensions.cs
index 2c657fcb..623f046c 100644
--- a/TransactionProcessor.BusinessLogic/Common/Extensions.cs
+++ b/TransactionProcessor.BusinessLogic/Common/Extensions.cs
@@ -8,6 +8,106 @@ namespace TransactionProcessor.BusinessLogic.Common
 {
     using System.Diagnostics.CodeAnalysis;
 
+    public static class Helpers
+    {
+        public static Guid CalculateSettlementAggregateId(DateTime settlementDate,
+                                                    Guid estateId)
+        {
+            Guid aggregateId = GuidCalculator.Combine(estateId, settlementDate.ToGuid());
+            return aggregateId;
+        }
+    }
+
+    public static class GuidCalculator
+    {
+        #region Methods
+
+        /// <summary>
+        /// Combines the specified GUIDs into a new GUID.
+        /// </summary>
+        /// <param name="firstGuid">The first unique identifier.</param>
+        /// <param name="secondGuid">The second unique identifier.</param>
+        /// <param name="offset">The offset.</param>
+        /// <returns>Guid.</returns>
+        public static Guid Combine(Guid firstGuid,
+                                   Guid secondGuid,
+                                   Byte offset)
+        {
+            Byte[] firstAsBytes = firstGuid.ToByteArray();
+            Byte[] secondAsBytes = secondGuid.ToByteArray();
+
+            Byte[] newBytes = new Byte[16];
+
+            for (Int32 i = 0; i < 16; i++)
+            {
+                // Add and truncate any overflow
+                newBytes[i] = (Byte)(firstAsBytes[i] + secondAsBytes[i] + offset);
+            }
+
+            return new Guid(newBytes);
+        }
+
+        /// <summary>
+        /// Combines the specified GUIDs into a new GUID.
+        /// </summary>
+        /// <param name="firstGuid">The first unique identifier.</param>
+        /// <param name="secondGuid">The second unique identifier.</param>
+        /// <returns>Guid.</returns>
+        public static Guid Combine(Guid firstGuid,
+                                   Guid secondGuid)
+        {
+            return GuidCalculator.Combine(firstGuid,
+                                          secondGuid,
+                                          0);
+        }
+
+        /// <summary>
+        /// Combines the specified first unique identifier.
+        /// </summary>
+        /// <param name="firstGuid">The first unique identifier.</param>
+        /// <param name="secondGuid">The second unique identifier.</param>
+        /// <param name="thirdGuid">The third unique identifier.</param>
+        /// <param name="offset">The offset.</param>
+        /// <returns>Guid.</returns>
+        public static Guid Combine(Guid firstGuid,
+                                   Guid secondGuid,
+                                   Guid thirdGuid,
+                                   Byte offset)
+        {
+            Byte[] firstAsBytes = firstGuid.ToByteArray();
+            Byte[] secondAsBytes = secondGuid.ToByteArray();
+            Byte[] thirdAsBytes = thirdGuid.ToByteArray();
+
+            Byte[] newBytes = new Byte[16];
+
+            for (Int32 i = 0; i < 16; i++)
+            {
+                // Add and truncate any overflow
+                newBytes[i] = (Byte)(firstAsBytes[i] + secondAsBytes[i] + thirdAsBytes[i] + offset);
+            }
+
+            return new Guid(newBytes);
+        }
+
+        /// <summary>
+        /// Combines the specified first unique identifier.
+        /// </summary>
+        /// <param name="firstGuid">The first unique identifier.</param>
+        /// <param name="secondGuid">The second unique identifier.</param>
+        /// <param name="thirdGuid">The third unique identifier.</param>
+        /// <returns>Guid.</returns>
+        public static Guid Combine(Guid firstGuid,
+                                   Guid secondGuid,
+                                   Guid thirdGuid)
+        {
+            return GuidCalculator.Combine(firstGuid,
+                                          secondGuid,
+                                          thirdGuid,
+                                          0);
+        }
+
+        #endregion
+    }
     public static class Extensions
     {
         /// <summary>
diff --git a/TransactionProcessor.BusinessLogic/EventHandling/TransactionDomainEventHandler.cs b/TransactionProcessor.BusinessLogic/EventHandling/TransactionDomainEventHandler.cs
index 9f57353c..7b55785f 100644
--- a/TransactionProcessor.BusinessLogic/EventHandling/TransactionDomainEventHandler.cs
+++ b/TransactionProcessor.BusinessLogic/EventHandling/TransactionDomainEventHandler.cs
@@ -242,7 +242,7 @@ private async Task HandleSpecificDomainEvent(TransactionHasBeenCompletedEvent do
                     Logger.LogInformation($"SettlementSchedule {merchant.SettlementSchedule}");
                     DateTime settlementDate = CalculateSettlementDate(merchant.SettlementSchedule, domainEvent.CompletedDateTime);
                     Logger.LogInformation($"Settlement Date {settlementDate}");
-                    Guid aggregateId = settlementDate.ToGuid();
+                    Guid aggregateId = Helpers.CalculateSettlementAggregateId(settlementDate, domainEvent.EstateId);
 
                     // We need to add the fees to a pending settlement stream (for today)
                     SettlementAggregate aggregate = await this.SettlementAggregateRepository.GetLatestVersion(aggregateId, cancellationToken);
@@ -259,6 +259,8 @@ private async Task HandleSpecificDomainEvent(TransactionHasBeenCompletedEvent do
             }
         }
 
+        
+
         private DateTime CalculateSettlementDate(SettlementSchedule merchantSettlementSchedule, DateTime completeDateTime)
         {
             if (merchantSettlementSchedule == SettlementSchedule.Weekly)
@@ -305,7 +307,7 @@ private async Task HandleSpecificDomainEvent(MerchantFeeAddedToTransactionEvent
                 return;
             }
 
-            Guid aggregateId = domainEvent.SettlementDueDate.ToGuid();
+            Guid aggregateId = Helpers.CalculateSettlementAggregateId(domainEvent.SettlementDueDate, domainEvent.EstateId);
 
             SettlementAggregate pendingSettlementAggregate = await this.SettlementAggregateRepository.GetLatestVersion(aggregateId, cancellationToken);
 
diff --git a/TransactionProcessor.BusinessLogic/Services/SettlementDomainService.cs b/TransactionProcessor.BusinessLogic/Services/SettlementDomainService.cs
index 49cd79e3..4e4b1860 100644
--- a/TransactionProcessor.BusinessLogic/Services/SettlementDomainService.cs
+++ b/TransactionProcessor.BusinessLogic/Services/SettlementDomainService.cs
@@ -22,7 +22,7 @@ public async Task<ProcessSettlementResponse> ProcessSettlement(DateTime settleme
         {
             ProcessSettlementResponse response = new ProcessSettlementResponse();
 
-            Guid aggregateId = settlementDate.ToGuid();
+            Guid aggregateId = Helpers.CalculateSettlementAggregateId(settlementDate,estateId);
 
             SettlementAggregate settlementAggregate = await this.SettlementAggregateRepository.GetLatestVersion(aggregateId, cancellationToken);
 
diff --git a/TransactionProcessor.IntegrationTests/Shared/SharedSteps.cs b/TransactionProcessor.IntegrationTests/Shared/SharedSteps.cs
index f4951f05..0007a15d 100644
--- a/TransactionProcessor.IntegrationTests/Shared/SharedSteps.cs
+++ b/TransactionProcessor.IntegrationTests/Shared/SharedSteps.cs
@@ -893,7 +893,7 @@ public async Task WhenIGetThePendingSettlementsTheFollowingInformationShouldBeRe
                 Int32 numberOfFees = SpecflowTableHelper.GetIntValue(tableRow, "NumberOfFees");
                 DateTime settlementDate = SpecflowTableHelper.GetDateForDateString(settlementDateString, DateTime.UtcNow.Date);
                 
-                var aggregateid = settlementDate.ToGuid();
+                Guid aggregateid = Helpers.CalculateSettlementAggregateId(settlementDate, estateDetails.EstateId);
                 await Retry.For(async () =>
                                 {
                                     SettlementResponse settlements =
diff --git a/TransactionProcessor/Controllers/SettlementController.cs b/TransactionProcessor/Controllers/SettlementController.cs
index 8dda2e69..372b227b 100644
--- a/TransactionProcessor/Controllers/SettlementController.cs
+++ b/TransactionProcessor/Controllers/SettlementController.cs
@@ -59,7 +59,7 @@ public async Task<IActionResult> GetPendingSettlement([FromRoute] DateTime settl
         {
             // TODO: Convert to using a manager/model/factory
             // Convert the date passed in to a guid
-            var aggregateId = settlementDate.Date.ToGuid();
+            Guid aggregateId = Helpers.CalculateSettlementAggregateId(settlementDate, estateId);
 
             Logger.LogInformation($"Settlement Aggregate Id {aggregateId}");