Sforno on Leviticus 25:8/ספורנו על ויקרא כ״ה:ח׳

1'והיו לך ימי שבע שבתות השנים וגו, the count of these years is not to be based on 12 lunar months per year only, but you will count hem as if regular years, the thirteenth month occurring at the same intervals as in your normal calendar considerations. The result will be that at the end of a Jubilee cycle, 49 solar years will have been completed. The same rules which apply to the sh’mittah calculation apply to the Yovel calculation. [since the farmer bases his activities on the seasons, and the seasons are controlled by the solar calendar, any other way of counting the 49 year cycle would throw the farmer’s schedule completely out of gear. Ed.]