Hawaii rental cars can be kept in increments of days, weeks, or by months.
A day constitutes 24 hours.

A week is generally 5, 6, or 7 discounted days, with 1 to 2 days being free.
One month, varying by car rental company equals 28 to 31 days.
Rentals periods may be negotiated for a total of 11 months.

Try your best to return your vehicle as close to its return time as possible. Charges can be added if you return your car just 60 minutes late. And, returning your vehicle too early, generally, one day, can also bring surcharges.