Why does he come back late and haggard?

Although he can travel in time to whenever he wants, he's human too. He's running on adrenalin and impatient and relieved to be alive, that he has to tell someone what happened. He wants to tell someone who can understand (his colleagues and friends), who are from his culture's context, from the time in which he was born) and he wants to tell it--Now! At once! Not a second later.

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The control panel on his machine only lists the month, day, and year, not the hour. He goes back to the date of the dinner, and has to guess about when to stop based on the degree of sunlight. The only way he could have assured that he would arrive prior to the dinner would have been to arrive on the previous date.

He comes back late for a simple reason-the time display on his Time Machine does not include the hours, minutes, and seconds. Just before Marty leaves 1955 in the first Back to the Future movie, he changes the destination time to arrive 10 minutes before he originally left 1985 to "go back early and warn" Doc that he would be assassinated by the Libyan terrorists. The Time Traveler should have included the hours, minutes, and seconds on the time display.