Last visit time changing in Active Topics...

The time will be lost if the servers application pool recycles and all the forums data wiped from the servers memory.

Contact your web hosts about the times that it's application pool recycles.

The cookie you refer to updates he user last visit date in the database, but not for the present session. This is required so that when the user next comes the forum knows the last time they visited the forum.

The application pool by default recycle every 1700 minutes, or something like that.

Most hosts however set it to recycle in the early hours of the morning when there is least activity. We set our shared hosting servers to recycle at 2am each morning.

If the server is rather overloaded or you have a troublesome users who brings down the application pool than the web host may set it to recycle as low as every hour to reduce the amount of memory the server requires, as we find on our servers that the application pool can use over 1 gig of memory.

The cookie you refer to is mainly used for Guests and non-logged in users, once a user logs in the last visit date is got from the database, so it works on multiple computers.