When we enter "T" or "t" on a date field, system will default to today's date. I think this functionality has been coded at PT_EDITSCRIPT (HTML Object).
We want to display yesterdays date when we enter "Y" or "y". I made necessary changes at PT_EDITSCRIPT but it didn't work for me when the date field is not in deferred processing. But for "T/t" it always work.
It worked for Y only on the delivered pages and the date field is checked off for "Allow Deferred Processing".
Cleared App Server, Web Server and client cache on every change.