Bonus is calculated daily on the "existing rate", and is given out monthly. Bonus Rate is compounded monthly. The existing rate is the bonus rate prescribed by Juno for the particular month.
If the daily bonus is 3% APY, then the calculation is as follows:
Daily Interest = Daily Balance at EOD * (3.00 divided by 365 divided by 100)
For example: if a transaction of $5000 is made on May 1, considering there are no other deposits or spends for the entire month, the daily bonus accrued at 3.00% APY will be $0.41.
5000 * (3.00 divided by 365 divided by 100) = $0.41/day
Till 31st May, the total bonus becomes 31 days * $0.41 = $12.71