Median

in column a1 to a5000 i have dates , multiple entries for different dates, in column b1 to b5000 i have values for each row, i need in column c to take the median of all the values in b that share the sme date in column a and to display this median on all rows in column c as i need to use these figures to do a calculation for each row individually, i am relatively new to excel 2010 but can find the average but need the median, i am stumped, please can somebody help it if it is possible