'Close of business' is a term many people use in their day to day working life without much thought. But what does it actually mean and should the term be used in contractual documentation?

Agreeing to get something done by 'close of business' is a phrase often used when flexibility is required as to the time a task will be completed. It makes it clear the task will be done that day, but not by a particular time. However, what does the term mean when it is included in a contract?