Calculate time between values

Hi all, 

I need to create a columnt that indicates how many days have passed between last two activities with the same client. Here's an example: 

So with the information in columns [ID] and [Date], I'd like to create the column [Days between activities]. In the above example the first two rows of table values would be first activities from new clients, thus the values in [Days between activities] being null (because there are no earlier activities). 

Any help would be greatly appreciated!