In addition to Mr. Omid,
I'm giving one more example:
Till today Personnel Number: 1234 Name is Manmohan Singh as of today, from tomorrow the same Person may change his name to Rahul Gandhi which is legally allowed.
Record Start date = Birth date because these details are valid from his DOB and Changed on a particular date, then we delimit the old record (Manmohan Singh) and enter the new valid record.