Find the OriginalDate field in Manage | Database | Fields and double click it to open Field Options. Set it up with an auto-enter calculation that is simply the name of the Date field. Leave the "Do not replace existing value..." check box selected and this field will automatically copy over the value of Date the first time that a date is entered, but will not copy it over if it is subsequently changed.
Thank you so much - that worked perfectly. I'm sorry i didn't work it out for myself.
Very kind of you.