I need to import Accounts and Opportunities into my sfdc org and I want to enter the created date and last modified date which is given by users in the excel sheet. How can I achieve this, I was told to raise a case with Salesforce. Please someone explain how and in which category I should create a case for this.
Yes, you can import records with created and last modified dates the first time. You need not to create a case with salesforce for this.
You need to enable this in Setup | Customize | User Interface | Enable "Set Audit Fields upon Record Creation" and "Update Records with Inactive Owners" User Permissions.
Please visit this link for more information :
Summary from that link:
To set values for audit fields on record creation, you must have the correct User permissions. See our "Enable Create Audit Fields" help article for more details. Since these fields are intended to be audit fields, it is only possible to set them on "create," not "update."
Date and Date Time values. Audit field values in a "Date" or "Date Time" field must be entered in the correct format to upload with Data Loader. See our "Date and Date Time values" help article for instructions. Custom Fields. You'll most likely need to create Custom Fields on your Standard and Custom Objects to transfer all field information from your previous CRM or Instance. See our "Create Custom Fields" help article for instructions. Remove Record IDs. Before you start an import to your new org, remove the Record Ids from the previous org from your export file.
Now that you have your groomed your exported CSV file for Salesforce, you'll need to Import it into your new Org