If the authority asked for the original date when you first arrive in the UK, put the actual date when you first arrive in the UK (see note below on what they expect as "first arrival"). The people processing your application will figure out that is more than five years ago, provided you have the relevant evidence.
Putting the date of five years counting back from now for the sake of convenience would not help anyone processing your application. You risk contradicting other evidences you are providing to support your claim that you have been here for the right period, and presenting a case of deception/fraud.
This is echoed by the declaration you will be signing when you fill in the EEA(PR) form (emphasis mine):
I hereby apply for a document certifying permanent residence / permanent residence card for myself and any family members included in this application. The information I have given in this form is complete and is true to the best of my knowledge.
I am aware that it is an offence to make a statement or representation which I know to be false
or do not believe to be true, or to obtain, or seek to obtain a document certifying permanent
residence or permanent residence card by means which include deception.
Note: The question is not clearly worded in the EEA(PR) form, though in the SET(O) form the Home Office elaborated the question a bit:
6.1 When did you (the main applicant) first enter the UK? This refers to the date you entered the UK at the beginning of the period of stay on which this application is based.