count the dates between an interval

Hello Community,

i have a table with Devices, Tasks and one or more dates for every task.
Now I want to calculate the first visit: The previous average date per task and the actual average date per task is between 30 days.
Then I want to calculate the second visit: The next average date per task and the actual average date per task is between 30 days.
I found a formula for the previous and the next date, but not how to calculate the first and second visits.

Would be thankful for every advice.

Best regards