First click on the date field to bring up the date picker, then as @Rahul_Unnikrishnan suggested.
you would amend the selector (aaname) to make it a variable, inside a click activity to select the relevant date.
The only thing you may need to look at is where is shows two dates of the same value, i.e. 30th March / 30th April etc.
You can use UiExplorer to understand more the fine tuning of the selector to make sure you pick the correct one.