Entitlement date is the date for company share registrar to look for the list of shareholders in CDS to determine the eligible shareholders.

Due to the fact share is not immediate transferred into our CDS whenever we trade share, it only incur after 2 days later, hence ex-date is "created' so that investors/traders in the market can know the last day they need to buy/have the share, in order to appear in the company share registrar book.

Summary,
Ex-date is the Cut off date for share trade.
Entitlement date - company registrar look for shareholders list for eligibility.