How to only keep entities in slot history within a certain number of dialog turns?

This is what I mean. Suppose I have the following dialog:

User:  What's the weather in Tokyo today?
Robot: it's has been rainy [city:tokyo, date:today]
User: what about tomorrow?
Robot: it will still be rainy. [city:tokyo, date:tomorrow]
User: what about next monday? 
Robot: it's going to be sunny. [city:tokyo, date:next monday]

User: Is there a japanese restaurant in the downtown?
Robot: yes [city:tokyo, date:next monday, cuisine:japanese, location:downtown].  

In this example, if I set the number of dialog turms=3, then it should forget all slot values that have been accumulated so far. So for the last robot response, the slot values should be: [cuisine:japanese, location:downtown].

Anything that is related with weather should have been discarded. Is there a way to destroy entities in slots after a certain number of dialog terns?