I have data in cell A1 which is linked to an external source and is constantly updating with a new value, is there a formula or macro I can use to show me the increase in value within another cell, here's an example

Cell A1 current value 100 auto updates to 180 after 15 seconds the result I want is the difference between the existing value 100 and the new value 180 i.e. 80 to be shown in another cell, is this possible? I've been looking at it for ages and cannot see how I can do it.

Any help would be greatly appreciated.

Gally