An exterior shot implies that nightfall is descending on New York City at the end of the workday (5pm). This would indicate that it is winter. However, most of the women employees are wearing short-sleeved or sleeveless warm-weather styles, and the CEO mentions that he plans to play golf the next day.
When all the secretaries are "working", they first change shoes from white to colored. After they make up their hair and stuff, a man yells "coffee break!" When they all get up to go, they're still wearing their white shoes.
During opening credits number, Robert Morse and a young fellow window washer board an electric scaffolding in exterior rooftop shot, but by next scene when scaffolding has descended a few floors, co-worker is now a much older man with much less hair.
Although Finch's rise to top of corporate world is supposedly pegged to sequential chapters of same-named title book he reads throughout movie, his progress through text isn't sequential - even near end of movie he is seen poring over beginning chapters for next bit of ladder-climbing advice.