In order to "substantially improve" a business property in an opportunity zone, you must make additions to the property's basis (i.e., expenditures that are capitalized), during any 30-month period, 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.