Here is some psuedo code for a correct leap year test calculation.

if year modulo 400 is 0

then is_leap_year

else if year modulo 100 is 0

then not_leap_year

else if year modulo 4 is 0

then is_leap_year

else

not_leap_year

Since this, and other date checking code, is built into the TSQL Date functions I recomend using the examples that have already been posted.