Cell converts a date entry to "=" and displays "01/00/00
 

A spreadsheet previously created changes a date entry to "=date typed in" and
displays "01/00/00" no matter what format has been applied to the cell.

Check in <view<names<define to see if there is a named cell called "date
typed in".
Are there underscores or anything between the words? I get an error message
when I put your equation in a cell and a "can't do it" note when I try to
name a cell "date typed in" I can't make it display "01/00/00".

"##" wrote:

A spreadsheet previously created changes a date entry to "=date typed in" and
displays "01/00/00" no matter what format has been applied to the cell.