What algorithm is used to calculate "new posts" on the forums?

Question

CMS

Hello!
On forums, usually, an icon is displayed to the left of the topic:
-there are new messages
-there are no new messages How
do they work? How is it better to consider whether there are new messages in the topic or not: since the last entry / or since the user opened the topic, or in some other way?
Who knows by what algorithm it works in Vbulletin & IPB & Simple Machine ?

Answer 1

In the database of users, each user has a field like last-visited. When the user connects, there is a comparison: if last-visited is less than the date of the post -> the post is new.
For guests, you can also cook additionally for cookies, as in the server, for example, the recipe is the same, only we gnaw cookies. Unreliable, but that's why they are guests.