As a first step you need to calculate the difference between the Date of Todayand Previous Posting Date. At the moment Anaplan does not have a formula integrated as Excel to retrieve Today's date (NOW()), so you need to import that everyday using Anaplan Connect or select it manually.
However, for the testing purposes, you can just create a module and manually select the date of today. Then as simple using (Today-PSTNG Date) you will get 'Days since Previous'. On the same line item (in blue print), you can change the summary method to Average, and get the result you are looking for. You can chose to calculate the average as well in a separate line item (use Select: Total).
Please find below a simple example I created in Anaplan based on my explanation. As well, there are other ways of calculating this, also depending of what do you want to do further with the result. If is statistics related, I recommend you to check for Statistics best practices on Anapedia.
However i still want to discuss a possible solution that might be useful, if not with your particular issue, then maybe with some other issues.
'Transactions' is a list that rolls up to 'Materials'.
'Rank' is a list, with a property 'Previous Rank' of 'Rank' format. (Please create this in a SYSTEM modules as part of DISCO)
'Transaction Module' has the 'Posting Date', and the general idea is to rank the 'Transactions' within each 'Material'. Then the 'Rank' number is converted to 'Rank' list, so that we can then send it to another module 'Find Transaction'.
'Find Transaction' will place the relevant 'Transactions' sequentially for each 'Material'. And referring to Rank.Previous Rank, we can then find the Previous Transaction.
Back in 'Transaction Module', we can then populate 'Previous Transaction', and subsequently populate 'Previous Posting Date' and then calculate 'Days since Previous'.
Changing the summary method of 'Days since Previous' will give you the average of 'Days since Previous' at Materials level.