I actually spoke to a Hertz agent yesterday regarding a reservation, and he told me that rates change at noon. Not always is there a difference but if it changes in middle of the day noon will be the cutoff point.
Many times the difference would be regarding weekday or weekend rates, it start Thursday at noon so if you the it at 11:30 it can be much more expensive then picking up at 12.
Same applies to taking a one day rental on Sunday, before 12 can be much cheaper then after 12.
Another thing I noticed is many times when reserving for 6 days I will get a weekly rate and when reserving for a day more - 7 days I'll get a weekly rate but will be more expensive, so you can reserve for 6 days and when you get to the counter ask them to change the return date to 7 days later and it'll still be the same cheaper weekly rate.
