When closing your financial year, the Tax Authorities look at the year in which you provided the delivered services. Work in December 2020 is included in the turnover of 2020, even if you do not send the invoice until 2021 and the customer will not pay until then.
In order to handle this properly for you, we will ask you in the 4th quarter about 'accrued revenue': these are activities that have already been (partly) completed in 2020, but have not yet been invoiced in 2020. You estimate this amount. If necessary, we will correct this later. This accrued turnover does NOT affect your VAT return or the VAT to be paid. We ask for this information to properly prepare your annual figures (part of your income tax return).
Do you have to send an invoice in January for work that took place in November or December? Then you write down the entire amount that you invoice in January.
Do you have to send an invoice in January for activities that take place partly in December and partly in January? Then you only state the amount for your activities in December (or part of it).