An interest rate differential is a difference in interest rate between two currencies in a pair.
It is not clear from the internet articles such as the one below which maturity length should be used for the interest rates in calculating the differential.
Let us use AUDUSD as an example. A person buys an AUDUSD currency futures contract that will expire in 1 month. Should we use 10-year, 1-year, 1-month government bond of the respective country to compute interest rate differential? Suppose we use 1-month government bond. What if 1-month government bond is available for U.S but not available for Australia?