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.
https://www.thebalance.com/what-is-an-interest-rate-differential-1344962
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?