How can I make an effective custom date default to only 1/01/1900?

I created a custom date field called Effective Date. But the user wants the date to immediately default to 1/01/1900. How can I make it default to 1/01/1900 on the Portal Screen and nobody can be able to edit this Effective Field.

The instruction is to run the preview the portal screen and immediately let the effective date "...Default to 1/01/1900 and do not allow edits to be made to this custom Effective Date field".

Thanks again,

alaso