Bond valuation using bndprice

Hi guys! I'm writing a program in which I need to determine the price of a number of bonds and bills. However, for some reason I get different prices when I try to calculate it "manually" and when I use the bndprice function. The reason is probably obvious, but I can't really see it. Any help would be much appreciated! An example:

%Create a simple T-bill example  
settlement='2010-01-01';
maturity='2012-12-31';
yield=0.05;
coupon=0;
period=0;
face=100;

%Convert dates to serial numbers  
settlement=datenum(settlement,'yyyy-mm-dd');
maturity=datenum(maturity, 'yyyy-mm-dd');

%bndprice uses semi annually compounded yields
yieldSemiAnnual=2*((1+yield)^(1/2)-1);

%Manual calculation. Use daysdif to calculate the number of days between  %settlement and maturity.
priceI=face/(1+yield*(daysdif(settlement,maturity,2)/360));

%Using bndprice
[a,b]=bndprice(yieldSemiAnnual,coupon,settlement,maturity,period,2);
priceII=a+b;

PriceI equals 86.799 and PriceII equals 86.395. What am I doing wrong?