What about online banking?  Is is deductible on the day you tell the bank to write and send it, or is it on the day they actually say they are going to send it?

If you authorize payment prior to year end it is in that year even if bank processes it later.  

Was this a Quicken question or a Turbotax question?  Just curious since I think the help forums have just  been merged.

The item is paid on the date you lose control of the money. It's that simple.

When you write a check, the day you drop that check in the mailbox you have effectively lost control of that money. The item is paid as of that date. Doesn't matter if you write the check on Dec 31st. If the date on that check is Dec 31st and you mail it on Jan 2nd, you effectively "lost control" of that money on Dec 31st. Doesn't matter if it two more months before the recipient actually cashes the check.

It was a tax question.

Hello, in your response the fist half makes sense but then its contradicted. Im still confused, is it the date written on the physical check or when you put in in outgoing mail in mailbox?

It is the date you actually put it in the mailbox. 

When calculating expenses, you will include the items you "put in the mailbox" before midnight on December 31, 2023 as expenses paid in 2023.  If you wrote the check on December 27th, but did not mail it until January 1st, 2024, you would need to wait until you file your 2024 return to deduct the expense.  

The check I am concerned with involves online banking.  From what you have said, I assume that the date that the bank mails the check, which I told them to write a week earlier, is the date for the deduction.  In your example, if I told them on December 27, 2023, to pay a charity, and they said they would write the check a week later, I could not take that deduction till 2024, since my saying to pay it is not the "mailbox."  Am I thinking correctly?

Correct. It would be when the bank drafted the check.