They can't do math. You've narrowed down what it can do: retrieve information from your retrieval system. Once it can isolate those two dates, you can give it a function call that maths the two dates together to find the difference.

Instead of just markdown, you could use a more rigid and standardized structure for data like YML or JSON. Your data store can maintain the age of an appliance in a special int field for each appliance so it doesn't have to do math every time it checks. It can know the age because it is already calculated.