Can you please include a screenshot with your "Final Shipping Time Weeks" line item? It looks like you are comparing it to your "Shipping Time Weeks" line item in your screenshots, which has 1 as the value, but your Final line item might have some other value and this is what you are including in your formula.
a) Confirmed PO Delivery = OFFSET(PO Submitted, -Final Shipping Time Weeks, 0)
b) Confirmed PO Delivery Test = POST(PO Submitted, Final Shipping Time Weeks)
With the above formulas, the results are as in Diagram 1. Am trying to understand why Confirmed PO Delivery is in Week 6 instead of 7 or is it supposed to be in Week 6. Essentially, am trying to understand the difference between OFFSET and POST.
It will move to Week 7 if the formula is changed to OFFSET(PO Submitted, -Final Shipping Time Weeks - 1, 0) -> Diagram 2
Post and Offset are very similar functions, however, they are going to give you very different results. Offset is going to give you the value from your selected line item x number of periods in the future or in the past. For example, in your line item Confirmed PO Delivery, week 6 is looking back 1 period (Final Shipping Time Weeks = -1) and resulting in your value of 1 since there is a 1 value in Confirmed PO Delivery in week 5.
Post, on the other hand, is going to push the value from your selected line item x number of periods in the future or in the past. For example, in your module, the 1 value in week 5 in your Confirmed PO Delivery line is being pushed 2 periods into the future because that is value in your second argument in your function (Final Shipping Time Weeks = 2). This function also allows you to sum values if they point to the same period. For example, if you also had a 1 in the Confirmed PO Delivery line item for week 6 it would be pushed forward 1 week into your week 7 and combine with your 1 value from week 5 to give you a value of 2 in week 7. Offset will not let you combine values in the same way.