When merging records, Salesforce always preserves the Oldest Create Date of the records that you are merging, no matter what date is displayed in the merge preview or on the master record.
When importing records using the Import Wizard, Cloudingo assigns a temporary create date as the beginning of time to records in the import file. Therefore records being imported are recognized as the Oldest Created record. However, when merging these records Salesforce will preserve the Oldest Create Date of the record already existing in Salesforce and not the import record.
When is this important? If you want the import record to be your master record, choose the oldest create date in your merge rules.