How do I edit the date and time assigned to a phone call log?

As it stands now, when I open a new log for a call, or make a change to an
existing call record, it defults to assigning it the current date/time as of
the editing.

There are times I wish to enter into the log some old conversations but the
date and time field are greyed out and dont allow me to change them from
today's date, to a previous date.

How do I change them to the date and time I choose?