From Drucker's The Effective Executive --
"One company chairman was absolutely certain that he divided his time roughly into three parts. One third he thought he was spending with his senior men. One third he thought he spent with his important customers. And one third he thought was devoted to community activities. The actual record of his activities over six weeks brought out clearly that he spent almost no time in any of these areas. These were the tasks on which he knew he should spend time—and therefore memory, obliging asusual, told him that these were the tasks on which he actually had spent his time. The record showed, however, that he spent most of his hours as a kind of dispatcher, keeping track of orders from customers he personally knew, and bothering the plant with telephone calls about them. Most of these orders were going through all right anyhow and his intervention could only delay them. But when his secretary first came in with the time record, he did not believe her. It took two or three more time logs to convince him that record, rather than memory, has to be trusted when it comes to the use of time."
Truer words have rarely been spoken -- measure what you're doing; your memory can't be trusted.