Hertz rentals are a maximum of 60 days, period. The online extension only works up to a month (e.g., you're got a 3 week reservation already, you can only extend another 9 days). Sometimes it doesn't work at all. Rates can change if you extend. Pretty easy to see online before you accept but ask first before accepting the rate in a phone call. Although on a phone call a recorded message says you might be charged $10 extra for not using the online extension, you never are.