When we said the Amidah in the winter - changes from Morid Hatal to Mashiv Haruach Umorid Hageshem and the other part is the brachah Ten Tal Umatar lebrachah. The first change is according with the Jewish Year and the 2nd it goes by the so called "secular year". The question is why we used the secular year?
We begin saying "mashiv haruach u'morid hageshem" at the end of the Sukkot holiday, which is when we pray for rain for the winter. In Israel we begin the daily prayer for rain (v'ten tal u'matar" two weeks later, when it can be assumed that holiday travellers are back in their homes. Outside of Israel rain is petitioned according to the solar season, when winter starts to set in.