The date the interest is posted to your account is not relevant.
Interest is charged on the balance outstanding every day. The sooner you make a repayment, the lower the balance will be, and so the less interest will be charged.
Let's say you are due to make a monthly repayment of €1,000 on 1st of the month, and you do not pay it until 15th month.
You will be charged additional interest as follows:
€1,000 @3.5% = €35 per year
€35/12 = €3 per month
€3/2 = €1.50 for half a month.
If you do it 12 times a year, the annual cost will be €18.
Brendan