Spanish to English Law/Patents Law: Taxation & Customs

el Ajuntament pasará a interpretar las tasas por ocupación con sillas y mesas por trimestres reales y no por trimestres naturales de manera que todas las solicitudes se abonarán por el período real de ocupación comprendido desde la fecha de inicio y la fecha de final independientemente de que éstas pueda abarcar el final de un trimestre natural y el principio del siguiente, lo que obligaría al interesado a pagar por un total de seis meses.

I understand the concept here, which is that previously if someone wanted to pay a tax to have tables and chairs on a bar terrace, for example, they would have to pay two calendar quarters if the period they chose overlapped two quarters, even though the actual period of the use of the public space was only 3 months (so for the period February to April you would actually have paid the first two whole quarters of the year from January to June). The change is that now you only pay the "real" quarter, but my problem is how we would express "trimestre real" compared with "trimestre natural" ("calendar quarter"?).

Thanks in advance. UK Eng please.