equities - how to calculate daily risk free rate using 13 week treasury bill

I want to calculate excess return for AAPL plus the S&P 500. I have computed monthly and daily logarithmic returns for every stock and for the market, I now need to calculate the risk free interest rate in order to be able to compute the excess return for every stock and the market.

Interest rates on three months T-Bills are a good proxy for the risk-free rate of return, but I have a lot of doubts on how to use data provided by Yahoo! Finance in order to compute the daily risk-free. I use the 13 week treasury bill (ticker: ^IRX) historical quotes provided by Yahoo! Finance. I divided t-bill rate by 100 and calculate the daily excess returns.

             t-bill         AAPL           Excess
2017-01-31    0.500     -0.00230205    -0.00730020
2017-01-30    0.495     -0.00262402    -0.00762402

However, I realized that monthly t-bill rate units are the same as daily rates.

              t-bill      AAPL          
Jan 2017      0.500      0.0477  
Dec 2016      0.480      0.0479 

I am making a mistake while computing daily and monthly t-bill rate units. The Federal Reserve (https://fred.stlouisfed.org/series/DTB3) also provide the same t-bill rates for daily and monthly frequency as in Yahoo Finance. How can I correctly calculate the excess returns? Thanks for any answer.