In a new document of the Calculation of VAT Liability type, you need to fill in the required On Date field in the upper area of form.
The "Calculation of VAT Liability (Create)" form
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In the Entity field, select an entity of your company for which this document is being prepared. The Entity field is available if the Multi-Company Accounting option is selected on: Administration > Settings > General Settings. You can select your personal default entity to appear in documents; for this, refer to Personal Settings. If you have not selected your default entity, the entity defined first will be used as the system default value.
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For descriptions of other buttons, see see Document Toolbar.