When the timeline is enabled for a base object, you can have a two-dimensional visibility into data. The two-dimensional visibility into data is based on the effective period and history of records. The effective period is the period of time for which a record is effective. You can define the effective period for a record by specifying the start date and the end date of the record. History is a past data event in the life of a record. To view the history of a record, specify a date from the past during which the record was effective.