During any 30-month period, you must make additions to the property's basis (i.e., expenditures that are capitalized) that exceed the property's basis at the start of the period. Note that the deadline for substantial improvement is not a fixed period from the date of purchase but a sliding window of 30 months. This means that you pick any point in time as your starting point (time A) and then test whether the amounts that you’ve added to basis over the next 30 months exceed the property’s basis at time A. Presumably, there must be an intent existing at the onset to meet this test, because QOF/QOZB qualification is relevant well before 30 months are up (even if it starts on the purchase date).
