Geordie613: You stated “and the one which is used for calculating leap years”. Are you referring to the secular leap year or the halachic leap year?
As for Pesach occurring before Tekufas Nissan please see Rabbi Dovid Heber’s article on the Star-K website (specifically footnote 16). I know that the moderator allows the links for hebrewbooks.org, I don’t know if they will allow the direct link to Rabbi Heber’s article. It originally appeared in the Winter 2008 edition of Kashrus Kurrents.