Of course you may add or subtract items as needed or change the time limits for your particular transaction. This little contract clause may save you from making a huge mistake down the road, so be sure to save this one for future reference!
Secret: Time is NEVER on your side when you are buying. Never forget that. Keep in mind that if you are looking to explore the viability of a purchase, a good starting point is the LOI. Once you get beyond the "overview" of what you and the seller are looking for, detail everything out in the purchase contract, and keep all of the above in mind when doing so.