Sheets or Excel - sum values in column a for each unique date and time in col b

I have two columns. One is a value, the other is a date and time:

A.                B.   01/01/21 5.15     301/01/21 5.15     201/01/21 5.15     401/01/21 5.30     201/01/21 5.30     3

I want the output to be:

A.                B.   01/01/21 5.15     901/01/21 5.30     5

Thanks!