When calculating simple interest, what must you do if you want to invest for months instead of years?

You must convert months to years. For example,
6 months = (6 months)*(1 year/12 months) = 6/12 = 1/2
6 months = 1/2 a year = 0.5 of a year

If we invest $500 at an interest rate of 2% for six months. That would mean
p = 500
r = 0.02 (decimal form of 2%)
t = 0.5 (it's easier to deal with decimals but only if they are terminating decimals)

Making the interest to be 
i = p*r*t
i = 500*0.02*0.5
i = 5

So you earn $5 in interest alone for that six month period.