It is pretty common knowledge that your broker helps to put all the appropriate documentation together such as the lease, work letter, and certainly the offers leading up to the lease.
What about after the lease is signed? Assuming you continue to engage your broker after the lease is signed, one important document you may find you are required to sign is the Commencement Date Amendment (CDA). This is a document that is usually part of the lease, but is purposely left blank. After you occupy the premises, all the blanks are filled in, including the actual size of the space and, most importantly, the actual start and termination dates.
Often, leases state a projected start date, but due to completion of TI's or other delays, the lease may actually start later. It is vital that both parties sign this document. We can't tell you how many times we get close to termination with a new client only to find no one is sure when that termination actually takes place. Landlords change, property managers change, and even tenant's management changes. Without the completed document, after all is said and done, all you have is the four corners of a blank page.
Have your broker make sure this is executed if you start any day, but the date stated on the lease.