Yes, it's supposed to be like that. :) In the information box at the top of the ad, it says
quote:
This advertisement will be available for bumping in _ days and _ hours.
Replies made before then will not alter the "Latest Comment" date.
The last comment date/time on the forum menu refers to the last valid bump (in this case the original post in the thread). It won't affect the date for bumping, as the thread will not be available for a bump until at least seven days after that last "bump".