According to Wikipedia, Treasury bills mature in 1 year or less to a fixed face value:
Treasury bills (or T-Bills) mature in one year or less.
Regular weekly T-Bills are commonly issued with maturity dates of 28 days (or 4 weeks, about a month), 91 days (or 13 weeks, about 3 months), 182 days (or 26 weeks, about 6 months), and 364 days (or 52 weeks, about 1 year). Treasury bills are sold by single-price auctions held weekly.
The T-bills (as Wikipedia says, like zero-coupon bonds) are actually sold at a discount to their face value and mature to their face value. They do not return any interest before the date of maturity. Because the amount earned is fixed at purchase, "return" is a more accurate term than "rate" when referring to a specific T-bill.
The "rate" is the difference between this return and the discount value you purchased it at.
So, yes, your rate of return is guaranteed.
T-notes (1-10 year) and T-bonds (20-30 year) also have an interest rate guaranteed, but have coupon payments (usually every 6 months), paying out a fixed amount of interest on the principal. (See more info on the same Wikipedia page.)
Because those bonds are not compounding the interest it pays out, but instead paying out every 6 months, you'd have to purchase new securities to create a compound return, changing your rate of return over time slightly as the rates for new treasury securities changes.