Timestamp field for a Dropdown filed - change in value

Hello, 

I created a timestamp calculated field to see the date when Lead Status changes from A to B, however the timestamp field does not return any values. Here is the formula I added: 

ifElse(equal($status,"SQL"),date(""),date(toString(now()))) 

I think the issue is with the (toString(now()))), that's why the field does not return any values - do you know what I could change it to? The goal is to get a full date DDMMYYYY when lead status changes from A to B. 

Thank you, 

Ada