diff --git a/src/services/dataService.ts b/src/services/dataService.ts
index 68f869e..01e81cc 100644
--- a/src/services/dataService.ts
+++ b/src/services/dataService.ts
@@ -23,20 +23,15 @@ export const dataService = {
           const response = await fetch('/contacts.json');
           const contactsData = await response.json();
           const contacts = contactsData.map((contact: RawContact) => {
+            const { createdAt, updatedAt, joinedAt, invitedAt, ...contactData } = contact;
             const processedContact: Contact = {
-              ...contact,
-              createdAt: new Date(contact.createdAt),
-              updatedAt: new Date(contact.updatedAt)
+              ...contactData,
+              createdAt: new Date(createdAt),
+              updatedAt: new Date(updatedAt),
+              joinedAt: joinedAt ? new Date(joinedAt) : undefined,
+              invitedAt: invitedAt ? new Date(invitedAt) : undefined
             };
             
-            // Convert optional date fields if they exist
-            if (contact.joinedAt) {
-              processedContact.joinedAt = new Date(contact.joinedAt);
-            }
-            if (contact.invitedAt) {
-              processedContact.invitedAt = new Date(contact.invitedAt);
-            }
-            
             return processedContact;
           });
           resolve(contacts);
@@ -126,17 +121,14 @@ export const dataService = {
           const response = await fetch('/groups.json');
           const groupsData = await response.json();
           const groups = groupsData.map((group: RawGroup) => {
+            const { createdAt, updatedAt, latestPostAt, ...groupData } = group;
             const processedGroup: Group = {
-              ...group,
-              createdAt: new Date(group.createdAt),
-              updatedAt: new Date(group.updatedAt)
+              ...groupData,
+              createdAt: new Date(createdAt),
+              updatedAt: new Date(updatedAt),
+              latestPostAt: latestPostAt ? new Date(latestPostAt) : undefined
             };
             
-            // Convert optional date fields if they exist
-            if (group.latestPostAt) {
-              processedGroup.latestPostAt = new Date(group.latestPostAt);
-            }
-            
             return processedGroup;
           });
           resolve(groups);
@@ -188,17 +180,14 @@ export const dataService = {
           const userGroups = groupsData
             .filter((group: RawGroup) => group.memberIds.includes(userId))
             .map((group: RawGroup) => {
+              const { createdAt, updatedAt, latestPostAt, ...groupData } = group;
               const processedGroup: Group = {
-                ...group,
-                createdAt: new Date(group.createdAt),
-                updatedAt: new Date(group.updatedAt)
+                ...groupData,
+                createdAt: new Date(createdAt),
+                updatedAt: new Date(updatedAt),
+                latestPostAt: latestPostAt ? new Date(latestPostAt) : undefined
               };
               
-              // Convert optional date fields if they exist
-              if (group.latestPostAt) {
-                processedGroup.latestPostAt = new Date(group.latestPostAt);
-              }
-              
               return processedGroup;
             });
           resolve(userGroups);