Leap Year concept: Why 1900 was not a leap year but 2000 was!

EArth takes 365.2425 days to revolve around the sun. We take into account only 365 days. And an year with 365 days is called as the Common year(february =28 days). The remaining 0.2425 we approximate as 0.25. Hence the error in the approximation is [-0.0075=(0.2425-0.25)]. Keep this negative error in mind because this will be used later.Now after 4 years 0.25 of a day becomes 0.25*4=1 day. So we add a day after 4 years in the month of february making it a month of 29 days. This year with 29 days in february or a total of 366 days in the year is called as the LEAP year. But due to the error of -0.0075 per year, which becomes considerably enough to be taken in to account after 100 years (i.e. -0.0075 * 100=-0.75 of a day), which again can be approximated to -1 day, we remove a day after 100 years, which makes a centurial year (which is not divisible by 400 ) a Common year (non leap year). But again in the approximation done above i have an error of 0.25 (=(-0.75) -(-1)) per 100 years. Now after 4 centurial years this error becomes exactly 1 day (=0.25*4=0.0075*400) which is added in the month of february. Hence every centurial year except for the ones which are divisible by 400 is a non-leap year.