Hi Melvin,

You can create a new date field to store the date on which the record has been moved to that particular state.Create another field to record the date of creation of the record. Use datetime library for calculating the measure you want from 2 days.As an example if you want to get the number of days between these 2 days see the below added example. 

Eg:  

creation_date = datetime.strptime(str(self.creation_date), "%Y-%m-%d")
state_change_date = datetime.strptime(str(self.state_change_date), "%Y-%m-%d")
days_difference = state_change_date - creation_date.days

Regards 

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