date and time needs to be auto populate when i insert data in another column.

Hi Team,

I have the function below in A1 column, below function I was tried to use because when I enter data in B column i need the data entered date and time in B column, however I have entered data in B1 and B2 column around 10 am and I got the exact date/time populated in A1 & A2 column, I am happy with that but after 2 hours I entered data in B3 and B4 column but the issue is data & time has been changed to current date from A1 to A4 column. Please advise and assist us on getting new function to auto populate the data and time in A column. No need macrosssssssssssss.

=if(COUNTA(b1)>0,Now(),"")


Regards
Girish