i need to Calculate the average days the tickets have been open in the past 30 days

i need to Calculate the average days the tickets have been open in the past 30 days from a specific number which is on sheet issue tracker chart B8.

When i change that date i need the formula to calculate + 30 days from that date then average it.

I tried make a table to get the individual number from each day then average it that way but i couldn't do that either.

Any help or advice would be appreciated.

UPDATED INCLUDES SPREEDSHEET ATATCHMENT