Hillel the Second calculated the Solar year to be 365 days 5 hours and 55 Mins, the actual amount is 365 days 5 hours and 49 mins.
Every year 6 mins are added to the year and over 1700 years almost 2 weeks have been added to the calender.
Tefukat Tisheri (First day of Fall) is supposed to be Oct 4, but its actually Sept 21
Tekufat Nissian (First Day of Spring) is supposed to be April 4, but its actually March 21. Causing in some years Pesech to fall out in Winter