What formula to use to count days after a date in a table?

I have a spreadsheet that contains a table. In this table there are many columns, but there are 9 that I am referencing for this. They all contain either dates or are blank, and from left to right in the row it is ALWAYS in chronological order. Think of them as dates a client comes in. Example may be column 1 has 2/4/2023, column 2 has 3/5/2024, column 3 has 5/25/2025, and columns 4 through 9 are blank (but won't be forever). I want to add another column that shows the amount of days since the most recent date they came in.

I tried Google and ChatGPT and nothing worked. It kept spitting out weird dates and when I converted it to numbers it was all wonky.

FYI - for the designation for the formula I tried both the column names and the cell numbers ranges - neither worked.

BONUS QUESTION - For clients names in each row that have a year or more on this counter, how would I create a list of these people? I am a Padawan of Excel 😂