Where, as a result tax, or more tax, becomes due a return or further return must be submitted within 30 days of the end of the one-year period concerned (see paragraph 3(4)).
Os, o ganlyniad, bod yn rhaid talu treth neu fwy o dreth, rhaid cyflwyno ffurflen neu ffurflen arall cyn pen 30 diwrnod ar ôl diwedd y cyfnod un flwyddyn dan sylw (gweler paragraff 3(4)).