Capture elapsed days chosen from date calendar

We need to capture and report on "elapsed days" for dates the customer chooses on a calendar and categorize the elapsed days so the business is able to understand how the customer interacts with the dates, for example: 50% of the clicks were within 6 months, 20% of the clicks were 18 months, etc.

We have discussed using an eVar and and event, however wanted to check here to see if there's a better way to implement this.

Question: is there a standard way to implement the above OR do we need to implement it with an eVar and event?