Update cell value to NOW() only when a specific cell is updated Without Apps Scripts

The idea is pretty simple, have a cell log date and time like NOW() when a cell in the same row or a specific cell is changed, But i didn’t find a solution without using Apps Scripts.

Example:

Ideally I’d like to use one formula to cover the whole Column, This is the best I came up with, it dones’t work as intended so I didn’t "beautify" it
IF(ISBLANK(AJ2),{"",""},IF(AI2="",{"",AI1},IF(AI2>NOW(),{"",AI1},))) In AH2

>Solution :

You may try something alike this:

=arrayformula(IF(A2:A,IF(B2:B="",NOW(),B2:B),""))

• Enable iterative calculation in sheets setting as in screenshot
• Format the Column B to timestamp format